docs: Clarify enable_nat_gateway vs single_nat_gateway

[excavator-matt]

Description

The previous documentation was vague with regards to enable_nat_gateway vs single_nat_gateway. These two statements could not be true at the same time, yet the two variables can both be true.

"Should be true if you want to provision NAT Gateways for each of your private networks"

"Should be true if you want to provision a single shared NAT Gateway across all of your private networks."

Motivation and Context

I was confused about this and I have seen other people be confused about this.

Breaking Changes

N/A

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request

N/A

[excavator-matt]

I am not sure who reviews, but it seems you have been reviewing some recent PRs @bryantbiggs. What do think? I can show you some public examples on where other people have been confused about this as well.

[bryantbiggs]

I don't think I've seen anyone confused by this

[bryantbiggs]

https://github.com/terraform-aws-modules/terraform-aws-vpc#nat-gateway-scenarios