doc: booti man-page

Provide a man-page for the booti command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 5754958..7dca913 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -15,6 +15,7 @@
 
    base
    bootefi
+   booti
    bootmenu
    button
    conitrace