SessionConfig.sliding_expiration description says 'on each access'

[allen0099]

Found during the docs audit (#360). The docs were corrected, but the field description is a string in code, which the audit could not touch.

fastapi_cachex/session/config.py:88-91:

sliding_expiration: bool = Field(
    default=True,
    description="Whether to refresh session expiry on each access",
)

The session is renewed only once less than session_ttl * sliding_threshold remains, not on every access (and get_session() saves only when it renews). The description shows up in IDEs, the JSON schema and the API reference. Suggested: "Whether to extend the session expiry once less than sliding_threshold of session_ttl remains". Also check the other SessionConfig field descriptions against the behaviour at the same time.