* Write stub as valid PHP
This works probably because it is not parsed by PHP but by something
more tolerant, but let's make it more valid (the final access type in
the signature is not valid) PHP anyway, that will raise fewer eyebrows.
* Document actual return type
See ca006e54e3/Zend/zend_exceptions.stub.php (L8)
* Mark all methods from Throwable as mutation free
Exception is the only possible class implementation of Throwable, and
all of its methods except __toString() are final.
See ca006e54e3/Zend/zend_exceptions.stub.php (L3-L25)Closes#3170