Rollup merge of #115087 - Nilstrieb:sizeassert, r=fee1-dead

Add disclaimer on size assertion macro

Sometimes people are inspired by rustc to add size assertions to their code and copy the macro. This is bad because it causes hard build errors. rustc happens to be special where it makes this okay.

For example, see #115028 (not sure whether they were directly inspired by this function), but I think I've also seen other cases.

Author: compiler-errors