Update clang-tidy documentation.

Summary:
Improve modernize-use-auto documentation (https://llvm.org/bugs/show_bug.cgi?id=24962).
Add documentation for modernize-make-unique.

Reviewers: klimek

Subscribers: cfe-commits, alexfh

Differential Revision: http://reviews.llvm.org/D13346

llvm-svn: 249017