add paragraph to sway-input man page
The added paragraph describes how to get button identifier and set it in
config.
@@ -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.
