doc: trigraph is removed in C++17

[e-kwsm]

preprocessor/doc/topics/file_iteration.html

Lines 97 to 113 in d899a71

	<div>
	All that we need now to perform a simple file iteration is to invoke the
	mechanism:
	</div>
	<div class="code">
	<pre>
	??=include BOOST_PP_ITERATE()
	</pre>
	</div>
	<div>
	(The <code>??=</code> token is a trigraph for <code>#</code>.&nbsp; I use the
	trigraph to make it clear that I am <i>including</i> a file rather than
	defining or expanding a macro, but it is not necessary.&nbsp; Even the digraph
	version, <code>%:</code>, could be used.&nbsp; Some compilers do not readily
	accept trigraphs and digraphs, so keep that in mind.&nbsp; Other than that, use
	whichever one you prefer.)
	</div>

preprocessor/doc/topics/local_iteration.html

Lines 84 to 96 in d899a71

	<div>
	Once these two macros are defined, the local iteration is initiated by <i>including</i> <b>BOOST_PP_LOCAL_ITERATE</b>().
	</div>
	<div class="code"><pre>
	??=include BOOST_PP_LOCAL_ITERATE()
	</pre></div>
	<div>
	(The <code>??=</code> token is a trigraph for <code>#</code>.&nbsp;
	I use the trigraph to make it clear that I am <i>including</i> a file rather than defining or expanding a macro, but it is not necessary.&nbsp;
	Even the digraph version, <code>%:</code>, could be used.&nbsp;
	Some compilers do not readily accept trigraphs and digraphs, so keep that in mind.&nbsp;
	Other than that, use whichever one you prefer.)
	</div>