man: clarify fi_cancel as time bounded

fi_cancel should return within a bounded period of time,
i.e. it cannot block indefinitely. Some providers may need
to perform additional actions that may not be "immediate".

Based on OFIWG discussion on 9/2.
man: update fi_cancel completion

Better wording.

There are 2 operations of concern here.  The original request being
canceled and the cancel operation itself.
Merge pull request #7 from sayantansur/manpage-updates

man: clarify fi_cancel as time bounded