Document opaque closures

[ulysses4ever]

Hello Julia devs! I was pondering about opaque closures but was a bit confused about the current state of documentation for the feature. Is there an expository description for it somewhere in the manual? A good starting point is in the apidocs in base/opaque_closure.jl but I can't see this text in the online manual either. And for the purpose of presentation this explanation would benefit from an example.

Further, there is an introductory text in the PR (#37849), maybe it also can be used to introduce the feature properly in the docs? The fact that the PR was closed (as opposed to merged) is also a bit off-puting: this gives me a second thought as to whether the design described there is what actually landed in the implementation.

[JeffBezanson]

This is an experimental feature and we're still figuring out exactly how it will work. For example #44008 is a significant change. There are also not yet really many uses for this outside of certain code transformations and optimizations.
Of course we should document it when it settles.