commit doc: document that -c, -C, -F and --fixup with -m error

Document that providing any of -c, -C, -F and --fixup along with -m
will result in an error. Some variant of this has been errored about
explicitly since 0c091296c0 ("git-commit: log parameter updates.",
2005-08-08), but the documentation was never updated to reflect this.

Wording-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -144,6 +144,9 @@ OPTIONS
 	Use the given <msg> as the commit message.
 	If multiple `-m` options are given, their values are
 	concatenated as separate paragraphs.
++
+The `-m` option is mutually exclusive with `-c`, `-C`, `-F`, and
+`--fixup`.
 
 -t <file>::
 --template=<file>::