Revert "Remove PROBLEMS section on double-buffering"

This reverts commit 366b2bc757.

We cannot be sure this problem is no longer relevant, since
its root cause was in software we don't control, and cannot
tell whether any of the versions where that problem happened
are still out in the wild.

etc/PROBLEMS

@@ -574,6 +574,21 @@ For example, simply moving through a file that contains hundreds of
 thousands of characters per line is slow, and consumes a lot of CPU.
 This is a known limitation of Emacs with no solution at this time.
 
+*** Display artifacts on GUI frames on X-based systems.
+
+This is known to be caused by using double-buffering (which is enabled
+by default in Emacs 26 and later).  The artifacts typically appear
+after commands that cause Emacs to scroll the display.
+
+You can disable double-buffering by evaluating the following form:
+
+  (modify-all-frames-parameters '((inhibit-double-buffering . t)))
+
+To make this permanent, add it to your ~/.emacs init file.
+
+Note that disabling double-buffering will cause flickering of the
+display in some situations.
+
 *** Self-documentation messages are garbled.
 
 This means that the file 'etc/DOC' doesn't properly correspond