Border checkbox disabling/enabling fix

Due to a lack of clarity in the wikitext documentation the border
checkbox wasn't set up right. This is the proper behavior: border is
allowed for all unframed images (frameless and basic) while the
framed images (frame and thumb) have it anyways. Wikitext
documentation was also clarified for prosperity.

Change-Id: I4dd4775da473d669781553796be99646240c13fe