Specify what version of multiple extends in docs

The first MVC of `extends` did not support a job having multiple
extends, this was added in GitLab 12.0.

closes https://gitlab.com/gitlab-org/gitlab-ee/issues/12733

doc/ci/yaml/README.md

@@ -2221,10 +2221,10 @@ spinach:
   script: rake spinach
 ```
 
-It's also possible to use multiple parents for `extends`.
-The algorithm used for merge is "closest scope wins", so keys
-from the last member will always shadow anything defined on other levels.
-For example:
+In GitLab 12.0 and later, it's also possible to use multiple parents for
+`extends`.  The algorithm used for merge is "closest scope wins", so
+keys from the last member will always shadow anything defined on other
+levels.  For example:
 
 ```yaml
 .only-important: