API and documentation feedback

[scottcode]

Below is feedback from a user:

I am not familiar with what is happening in your custom package dev_pkg and initially found it hard to figure out what is expected from custom function and how marries up to what udf_from_module is expecting. Maybe this would be clear for someone who is more familiar with Spark udf? Details of the API arguments (maybe I am missing?) and providing dev_pkg as example would be helpful for Spark newbies like myself. 

I ended up using the examples in the tests to figure out what is expected. There is no example of a function that takes 2 different spark_types. Is this possible?