Note in Lisp manual that mouse-face cannot change the text size.

* doc/lispref/text.texi (Special Properties):
* doc/lispref/display.texi (Overlay Properties): Note that mouse-face cannot
change the text size.

Fixes: debbugs:8530

doc/lispref/ChangeLog

+2011-08-09  Chong Yidong  <cyd@stupidchicken.com>
+
+	* text.texi (Special Properties):
+	* display.texi (Overlay Properties): Note that mouse-face cannot
+	change the text size (Bug#8530).
+
 2011-08-08  Chong Yidong  <cyd@stupidchicken.com>
 
 	* os.texi (Time of Day): Remove set-time-zone-rule, and recommend

doc/lispref/display.texi

@@ -1454,7 +1454,10 @@ specify just the foreground color or just the background color.
 @item mouse-face
 @kindex mouse-face @r{(overlay property)}
 This property is used instead of @code{face} when the mouse is within
-the range of the overlay.
+the range of the overlay.  However, Emacs ignores all face attributes
+from this property that alter the text size (e.g.  @code{:height},
+@code{:weight}, and @code{:slant}).  Those attributes are always the
+same as in the unhighlighted text.
 
 @item display
 @kindex display @r{(overlay property)}

doc/lispref/text.texi

@@ -3015,6 +3015,11 @@ near the character.  For this purpose, ``near'' means that all text
 between the character and where the mouse is have the same
 @code{mouse-face} property value.
 
+Emacs ignores all face attributes from the @code{mouse-face} property
+that alter the text size (e.g. @code{:height}, @code{:weight}, and
+@code{:slant}).  Those attributes are always the same as for the
+unhighlighted text.
+
 @item fontified
 @kindex fontified @r{(text property)}
 This property says whether the text is ready for display.  If