Improve OpenAPI Extensions docs

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

openapi_extensions.md

@@ -34,9 +34,10 @@ whole example, we use it to compose examples from other examples.
 
 ## Custom `x-pattern-format` key and custom formats 
 
-In JSON Schema, `format` is a property to convey semantic information about a
-schema. We define `x-pattern-format` as a key on the schemas under
-`patternProperties` with the same use as `format`, but that applies to the
-pattern of the property. We also define custom values for formats with the `mx-`
-prefix in `data/custom-formats.yaml`. Those values are recognized in the
-rendered specification and link to the definition of the format.
+In JSON Schema, [`format`](https://json-schema.org/understanding-json-schema/reference/string#format)
+is a property to convey semantic information about a schema. We define
+`x-pattern-format` as a key on the schemas under `patternProperties` with the
+same use as `format`, but that applies to the pattern of the property. We also
+define custom values for formats with the `mx-` prefix in
+`data/custom-formats.yaml`. Those values are recognized in the rendered
+specification and link to the definition of the format.