[css-fonts-4] Add a note about the generic emoji family and character support

Closes #2855

Author: litherum

css-fonts-4/Overview.bs

@@ -110,9 +110,7 @@ Font family: the 'font-family!!property' property</h3>
             Note that <<generic-family>> keywords cannot be quoted
             (otherwise they are interpreted as a <<family-name>>).
 
-            The following generic family keywords are defined:
-            ''serif'', ''sans-serif'', ''cursive'', ''fantasy'',  ''monospace'', ''system-ui'', ''emoji'', ''math'' and ''fangsong''.
-            See [[#generic-font-families]].
+            The set of generic family keywords is defined in [[#generic-font-families]].
 
     </dl>
 
@@ -370,6 +368,11 @@ Generic font families</h4>
 			food and drink, animals and plants,
 			or icons that represent emotions, feelings, or activities.
 
+            Note: Some user agents allow users to change the mapping of the ''emoji'' generic font
+            family by setting user agent preferences. Because the ''emoji'' generic font family is
+            intended to support emoji characters, user agents should encourage users to select
+            fonts for this generic font family which support those characters, if necessary.
+
         <dt id="math-def"><dfn>math</dfn>
         <dd>
             This font family is intended for use with mathematical expressions.