docs: clarify ModelSettings.truncation behavior

[MuhammedSuhaib]

docs: clarify ModelSettings.truncation behavior

• Expanded the docstring for ModelSettings.truncation.
• Documented the behavior of:
  • "auto": drops middle conversation items if input exceeds context window, request still succeeds and streaming continues normally.
  • "disabled": (default) request fails with 400 if input exceeds context window.
  • None: defers to provider default (typically "disabled").
• Aligns SDK docs with the official API reference.

[seratch]

The current one has the link to the document, so it should be sufficient. Also, we'd like to avoid duplicated list there for this property.