feat: document tg permit missing planfiles label

[ilya-hontarau]

Description of the change

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[TheMacies]

How about being more specific.

When a Spacelift stack configured with run-all points to a module that is not at the root of the Terragrunt module tree, execution may fail due to missing plan files from upstream dependencies. This happens because Terragrunt expects to manage the full dependency graph starting from the root module, and cannot correctly resolve external dependencies from a partial context.

To work around this limitation, you can add the stack label feature:terragrunt_permit_missing_planfiles, which allows Terragrunt to skip missing plan file errors. Be aware that you may still see errors from terraform plan commands in the logs—these are expected and will not interrupt the workflow.

[ilya-hontarau]

fair