Fix description for observer_time_step_size

[yifan-flex]

Note

Low Risk
Documentation-only change to a field description in outputs.py; no runtime or validation logic is modified in this diff.

Overview
Updates the AeroAcousticOutput.observer_time_step_size Pydantic field description so it matches actual constraints: a valid value must be greater than or equal to the CFD time step (replacing the incorrect “smaller than or equal” wording).

The description also documents that this setting cannot be changed on forked cases unless force_clean_start is enabled, aligning user-facing docs with fork/restart behavior next to the existing force_clean_start field.

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