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
- [ ] Evaluate / Prioritize Feature Request
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start by reading the discussion in #2081 and the BSIP-40 implementation context referenced in this issue. Create a dedicated documentation page covering the two stated limitations and future operation extensions, with completion shown by both caveats being clearly documented.

Written by the indexing model from the issue text.