Doxygen documentation errors

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Bug Description

Found some errors in Doxygen documentation for version 6.0.0:

• grouping in asset_issuer_permission_flags didn't work as intended, see https://doxygen.bitshares.org/namespacegraphene_1_1protocol.html#a31f22f43ceabd14325d69281eda006d7

• Documentation for the private match() functions is showing on the public match_result_type type, see https://doxygen.bitshares.org/classgraphene_1_1chain_1_1database.html#aaa234ff3fda171f0cf673fe338632599

Impacts Describe which portion(s) of BitShares Core may be impacted by this bug. 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 (please add below) : documentation

CORE TEAM TASK LIST

• [ ] Evaluate / Prioritize Bug Report
• [ ] Refine User Stories / Requirements
• [ ] Define Test Cases
• [ ] Design / Develop Solution
• [ ] Perform QA/Testing
• [ ] Update Documentation

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Fixed by #2659.