Add documentation about why people should almost never inherit from our exports.

[benlesh]

There are a lot of issues with doing this, and I think it needs to be in the official documentation that people should avoid doing it, and if they're doing it they're "voiding the warranty", so to speak.

High level:

• How they can end up relying on implementation details.
• How custom observables and subjects can break operators.
• etc.

Tangentially related #5431

I'll probably add this to the new guide in #5592