add paragraph to sway-input man page

The added paragraph describes how to get button identifier and set it in
config.

sway/sway-input.5.scd

@@ -92,6 +92,11 @@ For more information on these xkb configuration options, see
 *input* <identifier> scroll\_method none|two\_finger|edge|on\_button\_down
 	Changes the scroll method for the specified input device.
 
+*input* <identifier> scroll\_button <button\_identifier>
+	Sets button used for scroll\_method on\_button\_down. The button identifier
+	can be obtained from `libinput debug-events`.
+	If set to 0, it disables the scroll\_button on\_button\_down.
+
 *input* <identifier> tap enabled|disabled
 	Enables or disables tap for specified input device.