Extend documentation of how to test clang-tidy checks.

Summary:
The documentation assumed expertise with FileCheck; many clang-tidy check
authors may not have significant exposure to FileCheck, and so it's worth
spending a few more words here to spell things out.

Reviewers: alexfh

Subscribers: cfe-commits, JDevlieghere

Differential Revision: https://reviews.llvm.org/D28189

llvm-svn: 290838