Add more clear notes about the "mirror registry for Red Hat OpenShift" is not a production ready registry.

4.16 to 4.12 docs

The current documentation already states:
~~~
To run a production-grade registry service on such a cluster, you must 
create a separate registry deployment to install the first cluster. The mirror registry for Red Hat OpenShift addresses this need and is included in every OpenShift subscription
~~~

and

~~~
It is advised to leverage the mirror registry for Red Hat OpenShift to install a cluster that can host a production-grade, highly-available registry such as Red Hat Quay [...]
~~~

Add an important note to the "Mirror registry for Red Hat OpenShift introduction" emphasizing that the mirror registry is not intended to be a production ready registry.

In the "limitations" sections, include another one stating that additional non-documented limitations could be in place as the mirror registry is not intended for production use.