[AST][NFC] Small doc update for DeclContext
[AST][NFC] Small doc update for DeclContext

Factored out from https://reviews.llvm.org/D49729
following @erichkeane comments.

* Add missing classes in the list of classes
  deriving directly from DeclContext.
* Move the friend declarations together and
  add a comment for why they are required.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D49790


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@338887 91177308-0d34-0410-b5e6-96231b3b80d8

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