doc: include --guide option description for "git help"

Note that the ability to display an individual guide was always
possible. Include this in the update.

Also tell readers how git(1) can be accessed, especially for Git for
Windows users who do not have the 'man' command.  Likewise include a
commentary on how to access this page (Catch 22).

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-help.txt

@@ -8,31 +8,45 @@ git-help - Display help information about Git
 SYNOPSIS
 --------
 [verse]
-'git help' [-a|--all|-i|--info|-m|--man|-w|--web] [COMMAND]
+'git help' [-a|--all] [-g|--guide]
+	   [-i|--info|-m|--man|-w|--web] [COMMAND|GUIDE]
 
 DESCRIPTION
 -----------
 
-With no options and no COMMAND given, the synopsis of the 'git'
+With no options and no COMMAND or GUIDE given, the synopsis of the 'git'
 command and a list of the most commonly used Git commands are printed
 on the standard output.
 
-If the option '--all' or '-a' is given, then all available commands are
+If the option '--all' or '-a' is given, all available commands are
 printed on the standard output.
 
-If a Git subcommand is named, a manual page for that subcommand is brought
-up. The 'man' program is used by default for this purpose, but this
-can be overridden by other options or configuration variables.
+If the option '--guide' or '-g' is given, a list of the useful
+Git guides is also printed on the standard output.
+
+If a command, or a guide, is given, a manual page for that command or
+guide is brought up. The 'man' program is used by default for this
+purpose, but this can be overridden by other options or configuration
+variables.
 
 Note that `git --help ...` is identical to `git help ...` because the
 former is internally converted into the latter.
 
+To display the linkgit:git[1] man page, use `git help git`.
+
+This page can be displayed with 'git help help' or `git help --help`
+
 OPTIONS
 -------
 -a::
 --all::
 	Prints all the available commands on the standard output. This
-	option supersedes any other option.
+	option overrides any given command or guide name.
+
+-g::
+--guides::
+	Prints a list of useful guides on the standard output. This
+	option overrides any given command or guide name.
 
 -i::
 --info::