Fix AST matcher documentation.

Fix a bug in the matcher docs where callExpr(on(...)) was in the examples,
but didn't work (on() only works for memberCallExpr).

Fix a bug in the doc dump script that was introduced in r231575 when
removing a regexp capture without adapting the code that uses the
captures.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@245040 91177308-0d34-0410-b5e6-96231b3b80d8