Update docs for audit findings (correctness, safety, performance)

[mintlify]

Summary

Updates documentation to reflect 9 audit findings from jlucaso1/whatsapp-rust#460 covering correctness bugs, safety improvements, and performance optimizations.

Changes

• Signal protocol: Document sender key existence check using read-lock via SignalStoreCache and per-store independent flush during signal cache flush()
• Events/receiving messages: Document UndecryptableMessage event dispatch for group NoSenderKeyState decrypt failures
• Architecture/client: Document single cleanup_connection_state() call (removed duplicate in message loop)
• Architecture: Document per-chat message queue capacity of 500 (reduced from 10000)
• WebSocket handling: Document now_millis().max(0) guard for safe i64 to u64 timestamp casts
• Receiving messages: Document type-safe JID classification methods (is_group(), is_broadcast_list(), is_status_broadcast())