Doxygen warnings

**User Story**

There are quite some warnings when generating documentation via the `doxygen` tool.

Details: https://github.com/bitshares/bitshares-core/issues/2213#issuecomment-755413851.

**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)
- [x] Other: documentation

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

Start with the warning details in the linked issue comment and review the merged pull request #2324 to understand what work already moved there. Run the Doxygen documentation generation and identify the remaining warnings; done means the relevant warnings are resolved and the documentation build completes cleanly.

Written by the indexing model from the issue text.