Documentation: clarify that `-h` alone stands for `help`

We seem to be getting new users who get confused every 20 months or
so with this "-h consistently wants to give help, but the commands
to which `-h` may feel like a good short-form option want it to mean
something else." compromise.

Let's make sure that the readers know that `git cmd -h` (with no
other arguments) is a way to get usage text, even for commands like
ls-remote and grep.

Also extend the description that is already in gitcli.txt, as it is
clear that users still get confused with the current text.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-ls-remote.txt

@@ -28,7 +28,9 @@ OPTIONS
 	Limit to only refs/heads and refs/tags, respectively.
 	These options are _not_ mutually exclusive; when given
 	both, references stored in refs/heads and refs/tags are
-	displayed.
+	displayed.  Note that `git ls-remote -h` used without
+	anything else on the command line gives help, consistent
+	with other git subcommands.
 
 --refs::
 	Do not show peeled tags or pseudorefs like HEAD	in the output.

Documentation/gitcli.txt

@@ -120,6 +120,11 @@ usage: git describe [<options>] <commit-ish>*
     --long                always use long format
     --abbrev[=<n>]        use <n> digits to display SHA-1s
 ---------------------------------------------
++
+Note that some subcommand (e.g. `git grep`) may behave differently
+when there are things on the command line other than `-h`, but `git
+subcmd -h` without anything else on the command line is meant to
+consistently give the usage.
 
 --help-all::
 	Some Git commands take options that are only used for plumbing or that