Document incompatible overlay priority change

* doc/lispref/display.texi (Overlay Properties): Update re priority. 

* etc/NEWS: Related edit.

Fixes: debbugs:17234

2 files changed, 14 insertions, 6 deletions

diff --git a/doc/lispref/ChangeLog b/doc/lispref/ChangeLog
index 51f74f2eb1..060dca1adc 100644
--- a/doc/lispref/ChangeLog
+++ b/doc/lispref/ChangeLog
@@ -1,3 +1,7 @@
+2014-06-06  Glenn Morris  <[email protected]>
+
+	* display.texi (Overlay Properties): Update re priority.  (Bug#17234)
+
 2014-06-05  Glenn Morris  <[email protected]>
 
 	* package.texi (Package Archives): Mention signing packages.
diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index e93f9ab5a4..35282138b1 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -1515,9 +1515,9 @@ of them:
 @table @code
 @item priority
 @kindex priority @r{(overlay property)}
-This property's value determines the priority of the overlay.  No priority, or
-@code{nil}, means zero.  A non-nil and non-integer value has
-undefined behavior.
+This property's value determines the priority of the overlay.
+If you want to specify a priority value, use either @code{nil}
+(or zero), or a positive integer.  Any other value has undefined behavior.
 
 The priority matters when two or more overlays cover the same
 character and both specify the same property; the one whose
@@ -1527,9 +1527,13 @@ completely override the other value; instead, its face attributes
 override the face attributes of the lower priority @code{face}
 property.
 
-Currently, all overlays take priority over text properties.  Please
-avoid using negative priority values, as we have not yet decided just
-what they should mean.
+Currently, all overlays take priority over text properties.
+
+Note that Emacs sometimes uses non-numeric priority values for some of
+its internal overlays, so do not try to do arithmetic on the
+priority of an overlay (unless it is one that you created).  If you
+need to put overlays in priority order, use the @var{sorted} argument
+of @code{overlays-at}.  @xref{Finding Overlays}.
 
 @item window
 @kindex window @r{(overlay property)}