Plugin/Rewrite - review documentation of "append" (#1877)

* - ensure description of 'append' match what is encoded.

* - fix description based on review comments

plugin/rewrite/README.md

@@ -179,8 +179,8 @@ rewrite [continue|stop] name regex STRING STRING answer name STRING STRING
 
 Using FIELD edns0, you can set, append, or replace specific EDNS0 options on the request.
 
-* `replace` will modify any matching (what that means may vary based on EDNS0 type) option with the specified option
-* `append` will add the option regardless of what options already exist
+* `replace` will modify any "matching" option with the specified option. The criteria for "matching" varies based on EDNS0 type.
+* `append` will add the option only if no matching option exists
 * `set` will modify a matching option or add one if none is found
 
 Currently supported are `EDNS0_LOCAL`, `EDNS0_NSID` and `EDNS0_SUBNET`.