cli: document that -c/--count can be inconsistent with -l/--files-with-matches

This is unfortunate, but is a known bug that I don't think can be fixed
without either making `-l/--files-with-matches` much slower or changing
what "binary filtering" means by default.

In this PR, we document this inconsistency since users may find it quite
surprising. The actual work-around is to disable binary filtering with
the `--binary` flag.

We add a test confirming this behavior.

Closes #3131

tests/regression.rs

@@ -1545,6 +1545,7 @@ rgtest!(
     }
 );
 
+// See: https://github.com/BurntSushi/ripgrep/issues/3139
 rgtest!(
     r3139_multiline_lookahead_files_with_matches,
     |dir: Dir, _cmd: TestCommand| {