Fix issues found by Psalm

[muglug]

Found using this Psalm config on vimeo/psalm:dev-master.

Mostly just docblock improvements, with a few changes:

• I removed PHP_CodeSniffer\Files\File::getSuccessCount because it used an undefined property and seems never to have been called
• changed some DOMNodes to DOMElements because the called methods only existed on the subclass.
• added some properties to PHP_CodeSniffer\Tokenizers\Tokenizer, one ($numTokens) that's used in different functions and another ($scopeOpeners) that's referenced in that class but only defined in subclasses.