Fix test tags documentation (#1063)

* Fix test `tags` documentation

* Update test/README.md

Author: aphillips

test/README.md

@@ -52,8 +52,9 @@ for implementation-specific errors.
 
 ## Test Tags
 
-Some of the tests are for functionality that is not stable,
-i.e. is marked RECOMMENDED, OPTIONAL, or DRAFT.
+Some of the tests are for functionality that is optional or for functionality that is not yet stable.
+That is, the specification uses RFC2119 keywords such as SHOULD, SHOULD NOT, MAY, RECOMMENDED, or OPTIONAL,
+or the specification says that given functionality is DRAFT and not yet stable.
 Tests for such features have a `tags` array attached to them
 to mark the features that they rely on.
 This may include one or more of the following: