Document `unstable` APIs

[ajfriend]

We should add some comments on the unstable APIs.

Probably don't want to go too deep on documentation here. (Or maybe we're fine with whatever the auto-generated function signatures provide?)

Maybe note:

• that these exist and people should play with them
• that they are experimental and should not be relied upon, as they may change at any time
• if users do find them useful, we're more than happy to discuss adding them to the main APIs; users should file a ticket!
• even if users don't have an immediate request for these functions, we'd love to hear feedback and understand what works well and what doesn't

cc: @kylebarron