[clang-tidy] Add Abseil prefix to documentation

Summary: Adds the Abseil prefix to the list of prefixes in the documentation

Patch by Deanna Garcia!

Reviewers: aaron.ballman, hokein

Reviewed By: hokein

Subscribers: xazax.hun, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D51100

llvm-svn: 340412