[clang-tidy] Add a couple of paragraphs on run-clang-tidy.py to the docs.

Happy tidying.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@217373 91177308-0d34-0410-b5e6-96231b3b80d8
run-clang-tidy: Make clang-tidy path configurable and don't fail as horribly
when it's not around.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@217374 91177308-0d34-0410-b5e6-96231b3b80d8
Fixed a typo.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@217377 91177308-0d34-0410-b5e6-96231b3b80d8