Note caveats of `follow-tooltip' with system tooltips

* lisp/mouse.el (mouse-drag-and-drop-region): Turn off system
tooltips inside.
* src/xfns.c (Fx_begin_drag): Say that follow-tooltip doesn't
work with system tooltips.

lisp/mouse.el

@@ -3103,6 +3103,9 @@ is copied instead of being cut."
          ;; tooltip.
          (mouse-fine-grained-tracking t)
          (was-tooltip-mode tooltip-mode)
+         ;; System tooltips tend to flicker and in general work
+         ;; incorrectly.
+         (use-system-tooltips nil)
          ;; Whether or not some text was ``cut'' from Emacs to another
          ;; program and the cleaanup code should not try modifying the
          ;; region.

src/xfns.c

@@ -6884,7 +6884,8 @@ mouse buttons are released on top of FRAME.
 
 If FOLLOW-TOOLTIP is non-nil, any tooltip currently being displayed
 will be moved to follow the mouse pointer while the drag is in
-progress.
+progress.  Note that this does not work with system tooltips (tooltips
+created when `use-system-tooltips' is non-nil).
 
 This function will sometimes return immediately if no mouse buttons
 are currently held down.  It should only be called when it is known