misc-const-correctness not reported for C code

[firewave]

extern int cb();

void f1()
{
    int res = cb(); // no warning
    if (res) {}
}

https://godbolt.org/z/Tzrb1Yvxq

The documentation refers to C++ coding standards but there's no mention that the check will only be performed on C++ code.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

[njames93]

@JonasToth I feel like a quick fix is to just update the documentation to say this doesn't run on C code. Then we can look into adapting to make it work for C code, which I imagine is just a case of adding test cases and removing the language restriction.

[JonasToth]

The language restriction was added during review. const works different enough in C that it was removed.
I will provide a documentation update as known limitation

[JonasToth]

@njames93 i included the limitation in this patch: https://reviews.llvm.org/D132244
That is the separation of the documentation changes done for #56749