Allow floating views to appear at the mouse

doc/riverctl.1.scd

@@ -298,9 +298,11 @@ matches everything while _\*\*_ and the empty string are invalid.
 	  with make: _HP Inc._, model: _HP 22w_, and serial: _CNC93720WF_, the
 	  identifier would be: _HP Inc. HP 22w CNC93720WF_. If the make, model, or
 	  serial is unknown, the word "Unknown" is used instead.
-	- *position*: Set the initial position of the view, clamping to the
-	  bounds of the output. Requires x and y coordinates of the view as
-	  arguments, both of which must be non-negative. Applies only to new views.
+	- *position*: Set the initial position of the view, clamping to the bounds
+	  of the output. Requires x and y coordinates of the view as arguments, both
+	  of which must be non-negative. Optionally, the string "mouse" can appear
+	  as the only argument. In this case, the view will appear at the mouse's
+	  position. Applies only to new views.
 	- *dimensions*: Set the initial dimensions of the view, clamping to the
 	  constraints of the view. Requires width and height of the view as
 	  arguments, both of which must be non-negative. Applies only to new views.