[Parse] Document PrintStats, SkipFunctionBodies

Summary:
Add documentation for `clang::ParseAST` parameters `PrintStats` and
`SkipFunctionBodies`. Also, remove a tiny bit of trailing whitespace.

Reviewers: craig.topper, doug.gregor, erikjv

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36530

llvm-svn: 310609