Implement the correct rules for whether overridden methods should be documented

[timhunt]

Moodle coding style says that this is how it should be:

/** ... */
class base_class {
    /**
     * Document the purpose and contract of this method.
     */
    funcition do_something(): void {
    }
}

/** ... */
class sub_class extends base_class {
    funcition do_something(): void {
        // Just implement the purpopse defined in the base class, so there should not be a duplicate PHPdoc comment here.
    }
}

However, the PHPdoc checker currently gets this wrong, becuase it only analyses one file at a time.

However, now we rely more on class autoloading, we might finally be able to improve this, at least in the case where:

1. this file contains a single class definition.
2. This class extends classes/implements interfaces which are autoloadable.

Then, we can parse those files to get a list of public and protected methods, and we can use that in local_moodlecheck_functionsdocumented, to avoid giving warnigns about those funcitions. (Indeed, we could start issuing a warning if there is a comment there.)