Rename theme_fel_description to pair up with theme_form_element_label

Problem/Motivation

#2331941: Add class to configured description / help text extracted the rendering of #descriptions to a theme, theme_fel_description(). Now, that was a bit hasty decision as it is very close to theme_form_element_label() in functionality. They belong in pairs, so the hastily selected name is a bit weird in comparison:

• theme_form_element_label()
• theme_fel_description()

Proposed resolution

Rename theme_fel_description() to something that better pairs up with theme_form_element_label().Even though there is a awkward mismatch between $form['#title'] and theme_form_element_label() (title → label) the template is still called the same in D8, which makes this issue's argument stronger.

Possible candidates:

1. theme_form_element_description() which completely ignores this module's namespace. The logical name for it if it were implemented by core. Implemented on behalf of …
2. theme_fel_form_element_description. Same as above prefixed with our namespace.
3. theme_fel_element_description(). This follows the pattern for other theme replacements in this module.
4. theme_fel_description(). Keep it as is. When read out lout it is "theme_form_element_layout_description()" which is pretty close to the logical.