Implement map -release

doc/riverctl.1.scd

@@ -123,9 +123,11 @@ that tag 1 through 9 are visible.
 	but let the currently focused window in focus.
 	When set to _strict_ this is not the case. The focus will be updated on every cursor movement.
 
-*map* _mode_ _modifiers_ _key_ _command_
+*map* [-release] _mode_ _modifiers_ _key_ _command_
 	_mode_ is either “normal” (the default mode) or a mode created with
-	*declare-mode*. _modifiers_ is a list of one or more of the following
+	*declare-mode*.
+	If _-release_ is specified the mapping is executed on key release rather than key press.
+	_modifiers_ is a list of one or more of the following
 	modifiers separated with a plus sign:
 
 	- Shift