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

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

I propose to get this in first since doing a rename of the `Class` options might take some time. I'm having some hard time to find a good name that doesn't conflict with others, mostly due to `static` being such an overloaded keyword in C++ :) 

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