specials: Fix incomplete documentation of execute() param

specials: Fix incomplete documentation of execute() param

The parameter passed to the execute() method of each special page can
be null, and usually is. In some of these files this fact was already
mentioned in the comment, but not listed as a type.

In this patch I also remove comments that do not explain much. Saying
that the execute() method of a special page "executes a special page" or
is the "main entry point to a special page" is not super helpful. That's
usually what the documentation in the parent class shoudl explain.
We can add @inheritDoc tags in all these cases if you prefer. Please
tell me.

Change-Id: I1d811ab0c6d5c956e36f6a74120a425abc4332e6