Document library functions and referencable data

[mdarocha]

Currently, terranix offers a lot of library functions; some of these are documented on the website:

• https://terranix.org/functions.html
• https://terranix.org/eval-terranix-configuration.html

The purpose of this issue is to address library function documentation coverage:

• Find a total list of exported library functions
• Explore what can be documented here: Make resource and data referenceable terranix#43
• Document each and every one