[css-fonts-4] Add some motivation for generic font families

This is expository text.

Author: litherum

css-fonts-4/Overview.bs

@@ -233,6 +233,10 @@ Relationship Between Faces and Families
 <h4 id="generic-font-families">
 Generic font families</h4>
 
+    A generic font family is a font family which has a standard name (as defined by CSS), but which is an alias for an existing installed font family present on the system.
+
+    Note: Generic font families are intended to be widely implemented on many platforms, unlike arbitrary <<family-name>>s which are usually platform-specific names. They are expected to map to different fonts on different platforms. Authors may specify these generic family names if they desire their text to follow a particular design on many platforms, and are not particular about which specific font is chosen on those platforms.
+
     Each generic font family must always map to at least one matched font face.
     However, a single generic font family may be a composite face
     combining different typefaces based on such things as