[filter-effects] Add reference note to Filter region for shorthands s…

…ection in shorthand representations. Issue #23.

filter-effects/Overview.bs

@@ -3338,6 +3338,7 @@ Below are the equivalents for each of the filter functions expressed in terms of
 
 Where edge mode computes to ''filter/none'' for the 'filter' property and to ''duplicate'' for the CSS Image <<filter()>> function.
 
+Note: The <<blur()>> function may increase the UA defined filter region. See <a href="#filter-region-for-shorthands">Filter region for shorthands</a>.
 
 ### drop-shadow ### {#dropshadowEquivalent}
 
@@ -3354,6 +3355,7 @@ Where edge mode computes to ''filter/none'' for the 'filter' property and to ''d
     &lt;/filter&gt;
 </code></pre>
 
+Note: The <<drop-shadow()>> function may increase the UA defined filter region. See <a href="#filter-region-for-shorthands">Filter region for shorthands</a>.
 
 ## Filter region for shorthands ## {#filter-region-for-shorthands}