Documentation: Describe git-gui Tools menu configuration options.

Now git gui has a customizable Tools menu, so this adds
information about variables that are used to configure it.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -839,6 +839,57 @@ gui.blamehistoryctx::
 	Context` menu item is invoked from 'git gui blame'. If this
 	variable is set to zero, the whole history is shown.
 
+guitool.<name>.cmd::
+	Specifies the shell command line to execute when the corresponding item
+	of the linkgit:git-gui[1] `Tools` menu is invoked. This option is
+	mandatory for every tool. The command is executed from the root of
+	the working directory, and in the environment it receives the name of
+	the tool as 'GIT_GUITOOL', the name of the currently selected file as
+	'FILENAME', and the name of the current branch as 'CUR_BRANCH' (if
+	the head is detached, 'CUR_BRANCH' is empty).
+
+guitool.<name>.needsfile::
+	Run the tool only if a diff is selected in the GUI. It guarantees
+	that 'FILENAME' is not empty.
+
+guitool.<name>.noconsole::
+	Run the command silently, without creating a window to display its
+	output.
+
+guitool.<name>.norescan::
+	Don't rescan the working directory for changes after the tool
+	finishes execution.
+
+guitool.<name>.confirm::
+	Show a confirmation dialog before actually running the tool.
+
+guitool.<name>.argprompt::
+	Request a string argument from the user, and pass it to the tool
+	through the 'ARGS' environment variable. Since requesting an
+	argument implies confirmation, the 'confirm' option has no effect
+	if this is enabled. If the option is set to 'true', 'yes', or '1',
+	the dialog uses a built-in generic prompt; otherwise the exact
+	value of the variable is used.
+
+guitool.<name>.revprompt::
+	Request a single valid revision from the user, and set the
+	'REVISION' environment variable. In other aspects this option
+	is similar to 'argprompt', and can be used together with it.
+
+guitool.<name>.revunmerged::
+	Show only unmerged branches in the 'revprompt' subdialog.
+	This is useful for tools similar to merge or rebase, but not
+	for things like checkout or reset.
+
+guitool.<name>.title::
+	Specifies the title to use for the prompt dialog. The default
+	is the tool name.
+
+guitool.<name>.prompt::
+	Specifies the general prompt string to display at the top of
+	the dialog, before subsections for 'argprompt' and 'revprompt'.
+	The default value includes the actual command.
+
 help.browser::
 	Specify the browser that will be used to display help in the
 	'web' format. See linkgit:git-help[1].