help: controls-default: Key, reformat & alttext

The key for staying in place has been edited to "Begin" (i.e. without
numlock activated) to match all other key descriptions e.g. "PgUp", with
a note that some keyboards only show "5" on this key. Table reformatted.
Alt text added for images.

https://bugzilla.gnome.org/show_bug.cgi?id=726736