Add documentation to DefaultValue process plugin

Self-assigning for review.

1. +++ b/core/modules/migrate/src/Plugin/migrate/process/DefaultValue.php
   @@ -7,9 +7,38 @@
   + * value. The plugin sets the pipeline to the default value if the pipeline has
   + * no value (NULL, zero or empty string). The strict configuration key can be
   

   Let's remove these references to the pipeline -- they're confusing in the context of a single plugin. I think this should simply say "The plugin returns a default value if the input value is considered empty (NULL, FALSE, 0, '0', an empty string, or an empty array)".

2. +++ b/core/modules/migrate/src/Plugin/migrate/process/DefaultValue.php
   @@ -7,9 +7,38 @@
   + * Available configuration keys
   

   Should end with a colon.

3. +++ b/core/modules/migrate/src/Plugin/migrate/process/DefaultValue.php
   @@ -7,9 +7,38 @@
   + *     - false = apply default when pipeline value is empty().
   

   s/pipeline/input

4. +++ b/core/modules/migrate/src/Plugin/migrate/process/DefaultValue.php
   @@ -7,9 +7,38 @@
   + *     - true = apply default when value is NULL.
   

   "Apply default when input value is NULL."

5. +++ b/core/modules/migrate/src/Plugin/migrate/process/DefaultValue.php
   @@ -7,9 +7,38 @@
   + * Note that the pipeline starts with value NULL.
   

   Let's axe this sentence.

Added changed to class documentation.

Looking really good. One very minor change, then it's RTBC.

+++ b/core/modules/migrate/src/Plugin/migrate/process/DefaultValue.php
@@ -10,16 +10,16 @@
+ * configuration key can be used to set the default only when the pipeline

Let's change "pipeline" to "incoming".

Change added.

Full steam ahead!

Edit: Please ignore this message. Just getting a hang of writing patches. Thanks!

I applied patch at #7 first before patch at #9. Should I have not done that? Because #9 came back with following error messages:

error: patch failed: core/modules/migrate/src/Plugin/migrate/process/DefaultValue.php:7
error: core/modules/migrate/src/Plugin/migrate/process/DefaultValue.php: patch does not apply

This is on 8.4.x branch.

As a docs improvement, this can go in patch releases.

It'll be good to have these docs in the codebase. Just a couple small points:

1. +++ b/core/modules/migrate/src/Plugin/migrate/process/DefaultValue.php
   @@ -7,9 +7,38 @@
   - * This plugin sets missing values on the destination.
   + * Sets a default value.
   

   Sets a default value for what?

2. +++ b/core/modules/migrate/src/Plugin/migrate/process/DefaultValue.php
   @@ -7,9 +7,38 @@
   + * Available configuration keys:
   + *   - default_value: The fixed default value to apply.
   + *   - strict: (optional) Use strict value checking. Defaults to false.
   + *     - false = Apply default when input value is empty().
   + *     - true = Apply default when input value is NULL.
   

   The first list level is indented two spaces too far; the bullets should match the line above.

   Also, FALSE and TRUE should be all caps, and the = should be : instead to be formatted properly on API.d.o.

Thanks everyone!

Changes in response to code review.

Perfezione! (That's 'perfection' in Italian, according to Google Translate.)

Committed and pushed 559e61a to 8.4.x and e2b7797 to 8.3.x. Thanks!