docs: clarify mt5api operational scope

[dceoy]

Summary

• Clarify that mt5api exposes market data, account state, history, order validation, and narrow operational terminal endpoints over HTTP.
• Avoid implying that mt5api exposes live order-send endpoints or strategy orchestration.
• Align the feature list with the documented endpoint surface.

Testing

• Not run; documentation-only change.

[chatgpt-codex-connector]

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[claude]

The scope clarification and feature list expansion are a clear improvement over the previous description.

One pre-existing gap becomes more visible with this PR: the Mermaid architecture diagram (listing health · symbols · market · account · history · calc · trading) still omits the connection router. With this PR explicitly framing the operational surface, the diagram now reads as a complete inventory when it isn't. Worth adding connection to that list in a follow-up (or here).