Add class documentation for the bulk import models

Add documentation describing the purpose of these classes so that they
are more easily understood by developers new to the area.

Introduced in issue: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42978
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/45414

app/models/bulk_import.rb

 # frozen_string_literal: true
 
+# The BulkImport import model links together all the models required to for a
+# bulk import of groups and projects to a GitLab instance, and associates these
+# with the user that initiated the import.
 class BulkImport < ApplicationRecord
   belongs_to :user, optional: false
 

app/models/bulk_imports/configuration.rb

 # frozen_string_literal: true
 
+# Stores the authentication data required to access another GitLab instance on
+# behalf of a user, to import Groups and Projects directly from that instance.
 class BulkImports::Configuration < ApplicationRecord
   self.table_name = 'bulk_import_configurations'
 

app/models/bulk_imports/entity.rb

 # frozen_string_literal: true
 
+# The BulkImport::Entity represents a Group or Project that is going to be
+# imported during the bulk import process. An entity is nested under the a
+# parent group when it is not a top level group.
+#
+# A full bulk import entity structure might look like this, where the links are
+# parents:
+#
+#          **Before Import**              **After Import**
+#
+#             GroupEntity                      Group
+#              |      |                        |   |
+#     GroupEntity   ProjectEntity          Group   Project
+#          |                                 |
+#    ProjectEntity                        Project
+#
+# The tree structure of the entities will result in the same structure for the
+# imported Groups and Projects.
 class BulkImports::Entity < ApplicationRecord
   self.table_name = 'bulk_import_entities'
 
@@ -33,11 +50,17 @@ class BulkImports::Entity < ApplicationRecord
 
   def validate_imported_entity_type
     if group.present? && project_entity?
-      errors.add(:group, s_('BulkImport|expected an associated Project but has an associated Group'))
+      errors.add(
+        :group,
+        s_('BulkImport|expected an associated Project but has an associated Group')
+      )
     end
 
     if project.present? && group_entity?
-      errors.add(:project, s_('BulkImport|expected an associated Group but has an associated Project'))
+      errors.add(
+        :project,
+        s_('BulkImport|expected an associated Group but has an associated Project')
+      )
     end
   end
 end