Document BSIP-40 limits and caveats

[pmconrad]

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.)
• Security (the security of system or user data, etc.)
• 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

[MichelSantos]

We have an initial version of a precautions page