[clang-tidy][NFC][doc] Improve documentation for modernize-use-equals…

[carlosgalvezp]

…-delete

So the purpose of the check is more clear. Update examples code to show compliant code.

Fixes #65221

[PiotrZSL]

Maybe:

Identifies unimplemented private special member functions, recommends using '= delete' for them, and suggests relocating such deleted functions from the private to the public section.

Before the introduction of C++11, the primary method to effectively "erase" a particular function involved declaring it as private without providing a definition. This approach would result in either a compiler error (when attempting to call a private function) or a linker error (due to an undefined reference).

However, subsequent to the advent of C++11, a more conventional approach emerged for achieving this purpose. It involves flagging functions as "= delete" and retaining them within the public section of the code.

To prevent false positives, this check is only applicable within a translation unit where all other member functions have been implemented. The check will generate partial fixes by introducing "= delete," but the user is responsible for manually relocating it to the public section.

[PiotrZSL]

Other interesting thing is that currently this check is limited just to LangOpts.CPlusPlus, when it should be to LangOpts.CPlusPlus11. And in such case documentation could mention that requires C++11 or later.

[carlosgalvezp]

Yep, we could certainly change it to C++11 or later, but I think it's out of the scope for this NFC patch. Regarding documentation, I haven't seen that we document this detail in other checks, it seems like an implementation detail that could cause maintenance effort if documented.

[PiotrZSL]

LGTM

[PiotrZSL]

Do we want to sync comment above check class in code with first sentence in documentation ?

[carlosgalvezp]

Yes good point, will fix!

[PiotrZSL]

No more comments from me.