completion: improve docs for using __git_complete

It took me more than a few tries and a good lecture of __git_main to
understand that the two paragraphs really only refer to adding
completion functions for executables that are not called through git's
subcommand magic. Improve the docs and be more specific.

Signed-off-by: Roland Hieber <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rohieb

contrib/completion/git-completion.bash

@@ -31,15 +31,22 @@
 # Note that "git" is optional --- '!f() { : commit; ...}; f' would complete
 # just like the 'git commit' command.
 #
-# If you have a command that is not part of git, but you would still
-# like completion, you can use __git_complete:
+# If you have a shell command that is not part of git (and is not called as a
+# git subcommand), but you would still like git-style completion for it, use
+# __git_complete. For example, to use the same completion as for 'git log' also
+# for the 'gl' command:
 #
 #   __git_complete gl git_log
 #
-# Or if it's a main command (i.e. git or gitk):
+# Or if the 'gk' command should be completed the same as 'gitk':
 #
 #   __git_complete gk gitk
 #
+# The second parameter of __git_complete gives the completion function; it is
+# resolved as a function named "$2", or "__$2_main", or "_$2" in that order.
+# In the examples above, the actual functions used for completion will be
+# _git_log and __gitk_main.
+#
 # Compatible with bash 3.2.57.
 #
 # You can set the following environment variables to influence the behavior of