setcap: comment to help the user figure out why setcap is failing.

The file capabilities are not as expressive as process capabilities
(for a reason - see the NOTES section of 'man 3 cap_set_file').

The effective bits on a file under linux are captured by a single
boolean. As such attempting to partially set effective bits via the
more fully expressive process capability representation (cap_from_text)
sometimes yields an error. From now on, suggest that when the user
attempts to do this and an error occurs, the error might be such a
mismatch between effective and the other capability bits.

Signed-off-by: Andrew G. Morgan <morgan@kernel.org>