resource_gitlab_group_ldap_link: Update docs

[Jasper-Ben]

Description

Add a note about the ldap_provider not being the LDAP label value
and add information on how to receive the LDAP server name using a rake
task.

Fixes https://github.com/gitlabhq/terraform-provider-gitlab/issues/605

PR Checklist Acknowledgement

• I acknowledge that all of the following items are true, where applicable:
  • Resource attributes match 1:1 the names and structure of the API resource in the GitLab API documentation.
  • Examples are updated with:
    • A *.tf file for the resource/s with at least one usage example
    • A *.sh file for the resource/s with an import example (if applicable)
  • New resources have at minimum a basic test with three steps:
    • Create the resource
    • Update the attributes
    • Import the resource
  • No new //lintignore comments were copied from existing code. (Linter rules are meant to be enforced on new code.)

[timofurrer]

Awesome! Thanks @Jasper-Ben for stepping up and creating the PR 🎉

[github-actions]

This functionality has been released in v3.17.0 of the Terraform GitLab Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue. Thank you!