[clang-tidy] Added MacroDefiniton docs for readability-identifier-naming

Updates the docs to include `MacroDefinition` documentation. The docs are still missing `ObjCIVar` however I don't have a clue about how that looks in code. If someone wants to show the code block needed for the example I'll add that in too.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D80877