DOC: clarify what we mean by object oriented in pyplot api - #21093

I don't have access to the file you are linking. However in general I don't think there is any harm in repeating concepts in a few key places.

Sorry, I accidentally copied the link from a local doc build. Fixed above.

I think mentioning and cross-linking is better than half-touching a topic. It's easier to maintain. And we can keep the focus on the kind of documentation. The pyplot page here is primarily for reference documentation, but the added content is conceptual or maybe how to use.

We have had significant confusion from folks about statements like "the object oriented API is preferred..." then being confused why we use pyplot in almost every example. This modification guides people to the idea that there are currently two points to the pyplot module.

As discussed numerous times, the hope is to disambiguate these two concepts. But in the meantime I think it's helpful to be explicit and include explicit examples of the the two usages.