Documentation: describe -i/--info option to "git-help"

Option -i|--info for "git-help" is documented only in the new
"git-help.txt" man page, but this new man page is referenced
from the "--help" option documentation in the "git.txt" man page.
Signed-off-by: NChristian Couder <chriscool@tuxfamily.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-help.txt

@@ -7,7 +7,7 @@ git-help - display help information about git
 
 SYNOPSIS
 --------
-'git help' [-a|--all] [COMMAND]
+'git help' [-a|--all|-i|--info] [COMMAND]
 
 DESCRIPTION
 -----------
@@ -20,7 +20,8 @@ If the option '--all' or '-a' is given, then all available commands are
 printed on the standard output.
 
 If a git command is named, a manual page for that command is brought
-up. The 'man' program is used by default for this purpose.
+up. The 'man' program is used by default for this purpose, but this
+can be overriden by other options.
 
 Note that 'git --help ...' is identical as 'git help ...' because the
 former is internally converted into the latter.
@@ -32,6 +33,10 @@ OPTIONS
 	Prints all the available commands on the standard output. This
 	option superseeds any other option.
 
+-i|--info::
+	Use the 'info' program to display the manual page, instead of
+	the 'man' program that is used by default.
+
 Author
 ------
 Written by Junio C Hamano <gitster@pobox.com> and the git-list

Documentation/git.txt

@@ -104,6 +104,11 @@ OPTIONS
 	commands. If the option '--all' or '-a' is given then all
 	available commands are printed. If a git command is named this
 	option will bring up the manual page for that command.
++
+Other options are available to control how the manual page is
+displayed. See gitlink:git-help[1] for more information,
+because 'git --help ...' is converted internally into 'git
+help ...'.
 
 --exec-path::
 	Path to wherever your core git programs are installed.