[doctrings]: clarification for initial blade direction

[marcvivesmassana]

Note

Low Risk
Docstring-only changes clarifying how BETDisk.initial_blade_direction should be interpreted; no runtime logic or validation behavior is modified.

Overview
Clarifies the meaning and requirements of BETDisk.initial_blade_direction in volume_models.py, explicitly stating it is a global mesh-frame direction (not a disk-local frame), must be orthogonal to Cylinder.axis, and is required for unsteady BET Line when blade_line_chord > 0.

Updates the corresponding parameter docs in the BETDisk constructors to match this clarified definition.

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