rpc_api.md: address changes in stored procedures

Summary

• rpc_api.md: add .proc_wait_bucket_count — picodata@355e1483
• rpc_api.md: proofread and minor edits
• rpc_api.md: add .proc_replication_sync — picodata@641e8383
• rpc_api.md: edit .proc_replication_demote
• rpc_api.md: edit .proc_replication arguments — picodata@641e8383
• rpc_api.md: edit .proc_replication return value — picodata@0a8f88d2

picodata 24.5.0-350-g0a8f88d2
picodata 24.7.0-12-g641e8383
picodata 24.7.0-19-g355e1483

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• Close #395 (closed), #396 (closed)
• Cherry-pick to: none
• Staging:
  • ../architecture/rpc_api/#proc_replication
  • ../architecture/rpc_api/#proc_replication_demote
  • ../architecture/rpc_api/#proc_replication_sync
• Follow-up for picodata!1325 (merged)