[wp-docs] Custom Backgrounds

[Ken (WraithKenny)]

A section explaining some background on what the feature is would be nice.

There is potential for explaining the code side of Custom Backgrounds
(and Custom Headers). Plugins and Themes have an opportunity to extend
functionality in various hooks that hasn't been utilized very much. A
collection of the related hooks could be appropriate.

My favorite reason for looking at that particular page (and Custom
Headers) is to use it as an example for how to do a Settings page in a
php class (using "take_action" etc.) to make sure I don't leave
anything out (but I don't know how to utilize that in a codex page).