resource_gitlab_group_ldap_link: Update docs

Jasper Ben Orschulkorequested to merge
github/fork/Jasper-Ben/update_resource_gitlab_group_ldap_link_docs into main

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.)

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