remove false comments on ncplane_move_yx()

USAGE.md

@@ -914,7 +914,7 @@ ncplane_resize_simple(struct ncplane* n, int ylen, int xlen){
 
 // Move this plane relative to the standard plane, or the plane to which it is
 // bound (if it is bound to a plane). It is an error to attempt to move the
-// standard plane. Specifying a coordinate as -1 will hold it constant.
+// standard plane.
 int ncplane_move_yx(struct ncplane* n, int y, int x);
 
 // Move this plane relative to its current location. Negative values move up

doc/man/man3/notcurses_plane.3.md

@@ -279,6 +279,12 @@ to multiple ncplanes. So long as all threads are readers, multiple threads may
 work with a single ncplane. A reading function is any which accepts a **const
 struct ncplane**.
 
+A plane can be moved relative to its parent plane's origin with
+**ncplane_move_yx**. If the plane has no parent, the move is relative to
+the rendering area. A plane can be moved off-screen entirely, in which case
+it will not be visible following rasterization; it can also be partially
+off-screen.
+
 A plane has a virtual cursor; move it with **ncplane_cursor_move_yx**.
 Specifying -1 as one or both coordinates will hold that axis constant. Unless
 coordinates are specified for a call, action takes place at the plane's

include/notcurses/notcurses.h

@@ -1499,7 +1499,7 @@ API int ncplane_x(const struct ncplane* n) __attribute__ ((pure));
 
 // Move this plane relative to the standard plane, or the plane to which it is
 // bound (if it is bound to a plane). It is an error to attempt to move the
-// standard plane. Specifying a coordinate as -1 will hold it constant.
+// standard plane.
 API int ncplane_move_yx(struct ncplane* n, int y, int x);
 
 // Move this plane relative to its current location. Negative values move up