Document how and why d7_node_title_label works

+# Migrate Drupal 7's node title label

Should this have a period?

...changing the node title field's label using...

This is a difficult read for me. Would it be better as "node title's field label" ?

but only actually migrate

"actually" can probably be removed here to shorten up the sentence. It is a bit run on at the moment.

Source contains five content types, three have "title_label" set to "Title",
+#   two have custom strings.

This should be written as a more proper sentence. Beginning with "The" and having an "and" after the second comma.

I otherwise pass no judgement on whether this should be included. I trust Damian's opinion on its necessity.

@DamienMcKenna, thanks for making a patch. It really helps to see what docs you would like to see.

By chance I saw this issue. I would prefer this in the migration system component so that migrate maintainers will see it. That is, we review what is in the migration system component. Thinking about that for the moment.

On my first look at the patch I wondered why it was necessary at all, it is clear what the skip_on_empty is doing. Then I realized that I'm quite familiar with reading core pipelines so I should ignore that thought. Plus, adding documentation is almost always a good idea. So, moving on.

Then again, why this skip_on_empty and this migration? What makes this skip_on_empty different from the other 103 instances of skip_on_empty with a method of 'row' in core. Just thinking out loud.

1. +++ b/core/modules/node/migrations/d7_node_title_label.yml
   @@ -1,3 +1,19 @@
   +# Migrate Drupal 7's node title label
   

   I don't think this is necessary. The first two lines of the migration inform the reader that this is a migration for the Node title label for Drupal 7.

2. +++ b/core/modules/node/migrations/d7_node_title_label.yml
   @@ -1,3 +1,19 @@
   +# Drupal 7 allowed changing the node title field's label using a custom string
   +# stored in the "title_label" column in the "node_type" table. The default
   +# string is "Title".
   +#
   +# When this migration runs it will identify all of the "node_type" records as
   +# the total number of records to migrate, but only actually migrate records
   +# where the string is not equal to "Title".
   

   Can we shorten this? Something like, 'Migrate the title_label only for node types where the title_label is not the default value, 'Title'.

   And what do think about putting this in the pipeline just before 'label:'? I ask because I can't think of any migration yml that has a documentation block at the top, it is usually it is the pipeline and sometime the source or destination section.

3. +++ b/core/modules/node/migrations/d7_node_title_label.yml
   @@ -1,3 +1,19 @@
   +# Example scenario:
   

   Granted it is implied in the suggested description above, but I don't think an example is necessary.

Adding tag.

The Migrate Drupal Module was approved for removal in #3371229: [Policy] Migrate Drupal and Migrate Drupal UI after Drupal 7 EOL.

This is Postponed. The status is set according to two policies. The Remove a core extension and move it to a contributed project and the Extensions approved for removal policies.

The deprecation work is in #3522602: [meta] Tasks to remove Migrate Drupal module and the removal work in #3522602: [meta] Tasks to remove Migrate Drupal module.

Migrate Drupal will not be moved to a contributed project. It will be removed from core after the Drupal 12.x branch is open.

The Migrate Drupal Module and Migrate Drupal UI are deprecated and they are not in Drupal 12.0.0.

Issues for these modules should now be on the 11.x branch. And the changes are limited to critical and major bug fixes. Other changes are allowed at the discretion of the core Release Managers in consultation with the Migrate subsystem maintainers.