libfrog/xfs_scrub: improve iteration function documentation

libfrog/xfs_scrub: improve iteration function documentation

Between libfrog and xfs_scrub, we have several item collection iteration
functions that take a pointer to a function that will be called for
every item in that collection.  They're not well documented, so improve
the description of when they'll be called and what kinds of return
values they expect.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>