Better wording and $defaultMessages

When looking for where to put a reference for $defaultMessages, I found that the wording in Custom errors isn't very good. So I did my best to rephrase it better. I also added 2.1 method for adding errors to form components.
Merge pull request #92 from xpavp03/patch-3

Better wording, $form['element']->addError() and $defaultMessages