Fix ImmutableDictionary ArgumentException docs: clarify duplicate-key-same-value is a no-op

xml/System.Collections.Immutable/ImmutableDictionary.xml

@@ -935,6 +935,7 @@
         <summary>Enumerates a sequence of key/value pairs and produces an immutable dictionary of its contents.</summary>
         <returns>An immutable dictionary that contains the key/value pairs in the specified sequence.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TKey,TValue&gt;">
@@ -1049,6 +1050,7 @@
         <summary>Enumerates a sequence of key/value pairs and produces an immutable dictionary of its contents by using the specified key comparer.</summary>
         <returns>An immutable dictionary that contains the key/value pairs in the specified sequence.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TKey,TValue&gt;">
@@ -1126,6 +1128,7 @@
         <summary>Enumerates a sequence of key/value pairs and produces an immutable dictionary of its contents by using the specified key and value comparers.</summary>
         <returns>An immutable dictionary that contains the key/value pairs in the specified sequence.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TSource,TKey&gt;">
@@ -1179,6 +1182,7 @@
         <summary>Constructs an immutable dictionary from an existing collection of elements, applying a transformation function to the source keys.</summary>
         <returns>The immutable dictionary that contains elements from <paramref name="source" />, with keys transformed by applying <paramref name="keySelector" />.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TSource,TKey&gt;">
@@ -1242,6 +1246,7 @@
         <summary>Constructs an immutable dictionary based on some transformation of a sequence.</summary>
         <returns>The immutable dictionary that contains elements from <paramref name="source" />, with keys transformed by applying <paramref name="keySelector" />.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TSource,TKey,TValue&gt;">
@@ -1306,6 +1311,7 @@
         <summary>Enumerates and transforms a sequence, and produces an immutable dictionary of its contents.</summary>
         <returns>An immutable dictionary that contains the items in the specified sequence.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TSource,TKey,TValue&gt;">
@@ -1380,6 +1386,7 @@
         <summary>Enumerates and transforms a sequence, and produces an immutable dictionary of its contents by using the specified key comparer.</summary>
         <returns>An immutable dictionary that contains the items in the specified sequence.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToImmutableDictionary&lt;TSource,TKey,TValue&gt;">
@@ -1463,6 +1470,7 @@
         <summary>Enumerates and transforms a sequence, and produces an immutable dictionary of its contents by using the specified key and value comparers.</summary>
         <returns>An immutable dictionary that contains the items in the specified sequence.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
       </Docs>
     </Member>
   </Members>

xml/System.Collections.Immutable/ImmutableDictionary`2+Builder.xml

@@ -188,7 +188,7 @@
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentNullException">
           <paramref name="key" /> is null.</exception>
-        <exception cref="T:System.ArgumentException">An element with the same key already exists in the dictionary.</exception>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the dictionary.</exception>
         <exception cref="T:System.NotSupportedException">The dictionary is read-only.</exception>
       </Docs>
     </Member>

xml/System.Collections.Immutable/ImmutableDictionary`2.xml

@@ -1051,7 +1051,7 @@ This example shows how to create a new immutable dictionary by adding and removi
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentNullException">
           <paramref name="key" /> is <see langword="null" />.</exception>
-        <exception cref="T:System.ArgumentException">An element with the same key already exists in the <see cref="T:System.Collections.Generic.IDictionary`2" />.</exception>
+        <exception cref="T:System.ArgumentException">An element with the same key but a different value already exists in the <see cref="T:System.Collections.Generic.IDictionary`2" />.</exception>
         <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.IDictionary`2" /> is read-only.</exception>
       </Docs>
     </Member>