USB: document the 'u' flag for usb-storage quirks parameter

Commit d24d481b7d36 (usb-storage: Modify and export adjust_quirks so
that it can be used by uas) added the 'u' flag to the quirks module
parameter for usb-storage, but neglected to update the
documentation.  This patch adds the documentation.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: stable <stable@vger.kernel.org>  # 3.15+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/kernel-parameters.txt

@@ -3541,6 +3541,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 					bogus residue values);
 				s = SINGLE_LUN (the device has only one
 					Logical Unit);
+				u = IGNORE_UAS (don't bind to the uas driver);
 				w = NO_WP_DETECT (don't test whether the
 					medium is write-protected).
 			Example: quirks=0419:aaf5:rl,0421:0433:rc