[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

llvm-svn: 338887