fix: let's specify how terragrunt block works in case stack isn't terragrunt

[truszkowski]

Description of the change

spacelift-io/terraform-provider-spacelift#540 (comment)
The customer suggested that it's not obvious terragrunt block isn't applied for different (Terraform, Ansible... ) stack types.
Especially, it makes sense for monorepos, where we could mix different configurations.

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[adamconnelly]

I wonder if we'd be better adding a section called "Stack Vendor" or something like that, that explains the interaction between the vendor of your stack, and the config options. The reason I say this is that currently this info banner is inside the Terragrunt section, but in reality it's not a Terragrunt-specific piece of information because the same could apply for different vendor types.

The thing is it's a bit weird right now because the only vendor-specific options we expose currently are for Terragrunt. I'm just thinking it might be easier for people to understand the docs that way, and also if we exposed settings for other vendors in future the docs would still make sense.