Various improvements to the docstrings and user manual

[oskooi]

Various improvements to the docstrings and user manual:

• add documentation for the undocumented convenience functions meep.component_name and meep.direction_name
• fix an incorrect type hint in get_array
• fix incorrect usage of frequency $f$ vs. angular frequency $\omega = 2\pi f$ in a couple of equations in class PadeDFT
• use a formatting style for the types in class PadeDFT which matches the rest of the documentation