[css-fonts-4] Add note encouraging UAs to also implement platform-specific names for standard font families

As per @fantasai's comment in #4107 (comment)

Author: Litherum

css-fonts-4/Overview.bs

@@ -409,6 +409,15 @@ Standard font families</h4>
     Unicode range of the character, the content language of the containing
     element, user preferences, system settings, etc.
 
+    Note: These standard font families are expected to exist on different
+    platforms, but be backed by different fonts. However, some CSS authors might
+    prefer to handle this kind of platform-specific fallback themselves using
+    the 'font-family!!property' property. Therefore, user agents are encouraged
+    to also expose these standard font families by other, platform-specific,
+    font-family names, in addition to the standard names given here. Authors can
+    use these additional names to control font fallback themselves between different
+    platforms.
+
     <dl dfn-for="font-family,<generic-family>" dfn-type=value>
         <dt id="ui-serif-def"><dfn>ui-serif</dfn>
         <dd>