[docstrings] Inflow expressions

[marcvivesmassana]

Note

Improves documentation for expression-based boundary condition inputs.

• Adds Inflow example using expressions for spatially varying total_temperature and spec=TotalPressure(value=...), with velocity_direction.
• Expands TotalPressure docstring with both constant and expression examples, clarifying values must be nondimensionalized by operating-condition pressure.
• Docstring-only changes; no functional code modifications.

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