[clang-tools-extra] [clang-tidy][doc] Document that ClassMember* options apply only to static … (PR #227600)

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

> We should probably correct underlying Var->isStaticDataMember() issue instead..

I agree, happy to make that change otherwise. If we truly wanted this to apply only to static members it should be called `StaticClassMemberCase`.

https://github.com/llvm/llvm-project/pull/227600