gitcli: contrast wildcard given to shell and to git

People who are not used to working with shell may intellectually
understand how the command line argument is massaged by the shell
but still have a hard time visualizing the difference between
letting the shell expand fileglobs and having Git see the fileglob
to use as a pathspec.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/gitcli.txt

@@ -42,6 +42,23 @@ When writing a script that is expected to handle random user-input, it is
 a good practice to make it explicit which arguments are which by placing
 disambiguating `--` at appropriate places.
 
+ * Many commands allow wildcards in paths, but you need to protect
+   them from getting globbed by the shell.  These two mean different
+   things:
++
+--------------------------------
+$ git checkout -- *.c
+$ git checkout -- \*.c
+--------------------------------
++
+The former lets your shell expand the fileglob, and you are asking
+the dot-C files in your working tree to be overwritten with the version
+in the index.  The latter passes the `*.c` to Git, and you are asking
+the paths in the index that match the pattern to be checked out to your
+working tree.  After running `git add hello.c; rm hello.c`, you will _not_
+see `hello.c` in your working tree with the former, but with the latter
+you will.
+
 Here are the rules regarding the "flags" that you should follow when you are
 scripting git: