[ASTMatchers][NFC] Update comments on assorted `CXXMemberCallExpr` matchers.

Specifically:

* fixes the comments on `hasObjectExpression`,
* clarifies comments on `thisPointerType` and `on`,
* adds comments to `onImplicitObjectArgument`.

It also updates associated reference docs (using the doc tool).

Reviewers: alexfh, steveire, aaron.ballman

Differential Revision: https://reviews.llvm.org/D56849

llvm-svn: 353532