bug#35337: Document the recommended way to get the :0s out of -c output

[積丹尼 Dan Jacobson]

Boy they really blew it when they invented -c,
   (info "(grep) General Output Control")

  ‘-c’
  ‘--count’
       Suppress normal output; instead print a count of matching lines for
       each input file.  With the ‘-v’ (‘--invert-match’) option, count
       non-matching lines.  (‘-c’ is specified by POSIX.)

Who in the world ever wants to know about the zero match counts (:0)? ;-)
OK, well then add some extra switch for the rest of us... or at least the INFO
page should mention how the pros filter them out.

#1
time find * -name \*.html | xargs grep -l staticmap | xargs grep -c staticmap | 
head -n 3
location/directions/zaokeng.html:1
location/yaokeng_road.html:1
location/yaokeng_road2.html:1

real    0m0.015s
user    0m0.013s
sys     0m0.002s

#2
time find * -name \*.html | xargs grep -c staticmap | grep -v :0$   | head -n 3
comm/air/m750/trail.html:1
comm/air/m750/index.html:1
geo/house_numbering/taizhong/dongshi/index.html:1

real    0m0.017s
user    0m0.011s
sys     0m0.004s

(#1 is a big waste, but the real time is randomly a little shorter, as
it takes larger inputs so really see the difference.)

Anyway I've never ever wanted the :0s.

Wait! Maybe "-cl" or "-lc" will give us what we want!
Alas, they are just equivalent to "-l".
(As I suppose that fact is not documented, perhaps it could be given a
new (non-backwardly compatible, alas) definition...)