Document callable as possible

[greg0ire]

Custom string functions can either be a class string or a callable
returning the function.

orm/lib/Doctrine/ORM/Query/Parser.php

Lines 3636 to 3654 in 8f77012

	/**
	* @return FunctionNode
	*/
	public function CustomFunctionsReturningStrings()
	{
	// getCustomStringFunction is case-insensitive
	$functionName = $this->lexer->lookahead['value'];
	$functionClass = $this->em->getConfiguration()->getCustomStringFunction($functionName);
	assert($functionClass !== null);
	$function = is_string($functionClass)
	? new $functionClass($functionName)
	: call_user_func($functionClass, $functionName);
	$function->parse($this);
	return $function;
	}

As a follow up, I will make this more precise in 2.13.x (using class-string<FunctionNode>, and describing the callable more precisely.

[derrabus]

I think, the doc block of addCustomStringFunction() can be made more precise as well (class-string instead of just string).

[greg0ire]

@derrabus you're correct, and that's what I'm alluding to here:

I will make this more precise in 2.13.x

[derrabus]

PHPStan baseline needs an update.