Add documentation/comments

[nakamume]

Is your feature request related to a problem? Please describe.
None of the exported types, methods or functions seem to have any comments on them. It makes it super difficult to know:

1. What values to set for a type
2. Which method or function to call

Ex: specutils has couple of types with lots of fields and lots of methods but with zero comments.

Describe the solution you'd like to see
I love aws-sdk-go. Thanks to the comments, I've never felt any need for documentation.

Describe alternatives you've considered
Currently I'm just googling stuff for what values to set for a type. I've been playing trial-and-error to see if adding/removing a type field results in my desired output.