Ensure all public APIs in the msal4j package are covered by Javadoc comments

[Avery-Dunn]

Although most of our public APIs are documented, there are clearly gaps in many classes and methods: https://javadoc.io/doc/com.microsoft.azure/msal4j/1.21.0/index.html

To follow Java best-practices and better help our customers, all APIs should be covered by Javadoc comments.

To keep the PRs focused and manageable, this issue only applies to the com.microsoft.aad.msal4j package. The other packages in this repo (the broker at com.microsoft.aad.msal4jbrokers and persistence extensions at com.microsoft.aad.msal4jextensions) will be covered elsewhere.