[css-module-bikeshed] Note about required descriptors.

Author: tabatkins

css-module-bikeshed/Overview.bs

@@ -202,6 +202,8 @@ Initial: values
 For: @some-at-rule
 </pre>
 
+(A required descriptor can use `Initial: n/a</code>.)
+
 <h2 class=no-num id=priv-sec>Privacy and Security Considerations</h2>
 
 Issue: Make some considerations about privacy and security.