Document BSIP-40 limits and caveats

User Story As a user I want to be informed about limitations and caveats related to the BSIP-40 implementation

Impacts Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.

[ ] API (the application programming interface)
[ ] Build (the build process or something prior to compiled code)
[ ] CLI (the command line wallet)
[ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
[ ] DEX (the Decentralized EXchange, market engine, etc.)
[ ] P2P (the peer-to-peer network for transaction/block propagation)
[ ] Performance (system or user efficiency, etc.)
[ ] Protocol (the blockchain logic, consensus, validation, etc.)
[x] Security (the security of system or user data, etc.)
[x] UX (the User Experience)
[ ] Other (please add below)

Additional Context (optional) The discussion in #2081 brought up two points that should be put into the documentation:

Certain use-cases may not be possible despite the very general implementation, because they would require restrictions based on chain state not on the operation alone. E. g. a trading bot can be restricted to create orders in just one market, but it cannot be restricted to cancel only orders in one market.
Future protocol changes can add operation extensions that will not be covered by existing custom active authorities.

We should have a dedicated page to document these issues, and any related issues that might turn up in the future.

CORE TEAM TASK LIST