grep -lc: add note

[Dan Jacobson]

Add a note here:
`-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.
That
     "This option will not take effect if used with -l
     (--files-with-matches, as would only count to 1.)"