architecture/rpc_api.md: encoding peculiarities

Summary

• architecture/rpc_api.md: encoding peculiarities

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• Cherry-pick to: 24.4, 24.3, (24.2?)
• Staging: https://docs.binary.picodata.io/picodata/eivkov_fix_rpc_api
• See also:
  • tarantool-module#205
  • tarantool-module!479 (merged)