Adds the most requested doc for the AST matchers: a reference

of matchers, categorized by type and fully expanded for the
context in which they can be used.

I used a script to generate this documentation which I'll want
to be scrunitized by a code review before checking it in.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162225 91177308-0d34-0410-b5e6-96231b3b80d8