Add QA script to check for valid method names

[Girgias]

This is based on scripts/qa/check-valid-function.php.

However, I've walked into a conundrum because it turns out all functions use the <methodsynopsis> tag instead of <funcsynopsis> meaning that for the prototype of every function there is a usage of <methodname> which is currently being flagged.

I'm not totally sure what's the best way to approach this is in the script (because changing half the docs to be semantically correct, although nice, is not practical). So if anyone has ideas.