CLI config "host" block not documented

### Current Terraform Version

```
Terraform v0.11.11
```

### Use-cases

I am developing a private module registry and I hit a road block. It seems that module registries are forced to use HTTPS which means for local development I need to use a certificate.

So I generated my own root ca and cert for the registry server to use however there is no way to tell terraform that the certificate is valid.

I can add the CA to my system certificates, however this cert is just for local development so I rather not have to pollute my system certs with an insecure certificate authority.

### Attempted Solutions

Go reads the environment variable `SSL_CERT_DIR` for certificates, however this overrides the systems default which isn't ideal.

### Proposal

Add a `TF_REGISTRY_CA` environment variable or a `insecure = true` to the module definitions

The issue names no documentation file, test, or entry point. First reconcile the title’s missing host-block documentation with the body’s TLS/CA proposal, then locate the relevant CLI configuration page; what counts as done is unclear until the intended change is confirmed.

Written by the indexing model from the issue text.