php
Fix Throwable note to mention that Error can be extended (fixes #1647).
But see https://github.com/php/doc-en/issues/1647#issuecomment-1165439900 f. I'm really unsure whether userland code should extend Error.
It might be a good idea to add a note that says although it is possible to extend Error this is in general a bad idea as those should only refer to programming/engine errors.
How sure are you that userland code shouldn't extend Error? It doesn't seem that common, but some popular projects are doing it: https://grep.app/search?q=extends%20%5CError&words=true&filter[lang][0]=PHP (a lot of that is stubs and polyfills, but about half of them seem to be actual userland extensions of Error).
If Error really shouldn't be extended I'll add a note to the Error documentation as well, as I mentioned here I didn't think the documentation was clear on that. In retrospect, allowing Error to be extended in userland makes sense for polyfills, but it should be more clear that it's not intended for general use.