Add doc-strings to ported code

[dpsimpson]

What behavior of the library made you think about the improvement?

This library is very light on doc-strings, which isn't ideal for people getting into the code base (and isn't good practice anyway).

How would you like it to behave?

Every function should have at least a basic docstring, having

• Inputs
• Outputs
• Purpose / Usage (if complex) / High-level method (if complex)