Update examples for rules

The example rules may cause duplicate
pipelines and don't always make things
clear, so update for clarity and prevent
doubled pipelines if the examples are
copied as-is.

doc/ci/merge_request_pipelines/index.md

@@ -207,7 +207,7 @@ The variable names begin with the `CI_MERGE_REQUEST_` prefix.
 ### Two pipelines created when pushing to a merge request
 
 If you are experiencing duplicated pipelines when using `rules`, take a look at
-the [key details when using `rules`](../yaml/README.md#key-details-when-using-rules),
+the [important differences between `rules` and `only`/`except`](../yaml/README.md#differences-between-rules-and-onlyexcept),
 which will help you get your starting configuration correct.
 
 If you are seeing two pipelines when using `only/except`, please see the caveats