Bring conformance doc in line with clarification to use of region nam…

…es/area_types to allow use of flag_values and flag_meanings as per discussion in cf-convention#198

conformance.adoc

@@ -201,8 +201,9 @@ name modifier.
 name table.
 * The legal values for the standard name modifier are contained in
 Appendix C, Standard Name Modifiers.
-* If a variable has a **`standard_name`** of **`region`** or **`area_type`**, it must have value(s) 
-from the permitted list.
+* If a variable has a **`standard_name`** of **`region`** or **`area_type`**, it must
+either have value(s) from the permitted list or, if flags are being used, the
+**`flag_meanings`** attribute must contain values from the permitted list.
 
 *Recommendataions:*