Documentation and rules for form validations

This has been created because MDL-42270 revealed various inaccuracies and misses in the moodleforms documentation and implementation.

1) http://docs.moodle.org/dev/lib/formslib.php_Validation is missing completely about how to add built-in validation rules for groups.

2) The documented strings suggested for use with addRule methods have been 100% ignored and, instead, other strings are being used. Decide.

3) We should cover all the rules with some test, both server side (unit tests) and browser side (acceptance).

That's all, let's manage the whole thing from here. Ciao