doc: fatinfo man-page

Provide a man-page for the fatinfo command.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

doc/usage/index.rst

@@ -28,6 +28,7 @@ Shell commands
    exception
    exit
    false
+   fatinfo
    for
    load
    loady