git-prompt.sh: update PROMPT_COMMAND documentation

The description of __git_ps1 function operating in two-arg mode was
not very clear.  It said "set PROMPT_COMMAND=__git_ps1" which is not
the right usage for this mode, followed by "To customize the prompt,
do this", giving a false impression that those who do not want to
customize it can get away with no-arg form, which was incorrect.

Make it clear that this mode always takes two arguments, pre and
post, with an example.

The straight-forward one should be listed as the primary usage, and
the confusing one should be an alternate for advanced users.  Swap
the order of these two.

Acked-by: Simon Oosthoek <s.oosthoek@xs4all.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -10,14 +10,20 @@
 #    1) Copy this file to somewhere (e.g. ~/.git-prompt.sh).
 #    2) Add the following line to your .bashrc/.zshrc:
 #        source ~/.git-prompt.sh
-#    3a) In ~/.bashrc set PROMPT_COMMAND=__git_ps1
-#        To customize the prompt, provide start/end arguments
-#        PROMPT_COMMAND='__git_ps1 "\u@\h:\w" "\\\$ "'
-#    3b) Alternatively change your PS1 to call __git_ps1 as
+#    3a) Change your PS1 to call __git_ps1 as
 #        command-substitution:
 #        Bash: PS1='[\u@\h \W$(__git_ps1 " (%s)")]\$ '
 #        ZSH:  PS1='[%n@%m %c$(__git_ps1 " (%s)")]\$ '
-#        the optional argument will be used as format string
+#        the optional argument will be used as format string.
+#    3b) Alternatively, if you are using bash, __git_ps1 can be
+#        used for PROMPT_COMMAND with two parameters, <pre> and
+#        <post>, which are strings you would put in $PS1 before
+#        and after the status string generated by the git-prompt
+#        machinery.  e.g.
+#           PROMPT_COMMAND='__git_ps1 "\u@\h:\w" "\\\$ "'
+#        will show username, at-sign, host, colon, cwd, then
+#        various status string, followed by dollar and SP, as
+#        your prompt.
 #
 # The argument to __git_ps1 will be displayed only if you are currently
 # in a git repository.  The %s token will be the name of the current