Working with terraform creating modules I would like to have documentation so I've used terraform-docs.
It is tool to generate documentation based on terraform code contains:

• input variables
• outputs
• providers
• requirements

Usage is very simple:
terraform-docs markdown table . > README.md

so you get generated documenation in README.md file in markdown format with tables based on .tf files in current directory.

I've created zsh plugin to completation commands and flags. I've decided to share my code in Github:

https://github.com/jsporna/terraform-docs-zsh-plugin

PS
Tool itself contains completetion command to setup bash and zsh but it was added much later than I've created my code.