[php-langspec][PR #229] - Change isset expression

You can view the Pull Request on github:
https://github.com/php/php-langspec/pull/229

Comment by nikic:
This syntax was added as part of https://wiki.php.net/rfc/trailing-comma-function-calls, which also added support for `unset($foo,)` and `call($foo,)`. Maybe you would like to add those two as well?