CStr::from_ptr should be documented to not allow strings bigger than isize::MAX

[KamilaBorowska]

In #53784, slice::from_raw_parts was documented to not allow slices larger than isize::MAX bytes. However, CStr::from_ptr is not documented to have the same requirement, despite allowing the user to obtain a slice larger than isize::MAX bytes with its to_bytes method.

[elichai]

I think this issue can be closed Now that #64326 is merged :)