[clang-tidy] Improve google-objc-function-naming diagnostics 📙

Summary:
The diagnostics from google-objc-function-naming check will be more
actionable if they provide a brief description of the requirements from
the Google Objective-C style guide. The more descriptive diagnostics may
help clarify that functions in the global namespace must have an
appropriate prefix followed by Pascal case (engineers working previously
with static functions might not immediately understand the different
requirements of static and non-static functions).

Test Notes:
Verified against the clang-tidy tests.

Reviewers: benhamilton, aaron.ballman

Reviewed By: benhamilton

Subscribers: MyDeveloperDay, xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D55482

llvm-svn: 349123