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

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

> LGTM, by the way did you check that in tests we also check `static` methods for ClassMethodCase and friends?

Yep. I actually notice that the warning message looks like:

```
invalid case style for class constant
invalid case style for class constexpr
invalid case style for class member
```

So it seems the naming was quite deliberate. "class constant" is a "constant of the class", not of an "instance" of the class. Maybe in another language (Java?) this naming is more common. 

I wonder if I should update the warning messages as well or if it will make things more inconsistent.

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