Add documentation to Config destination plugin

Here's a first draft, I have added:

• available configuration key(s)
• destination properties
• a simple example
• a second example incorporating the translation config key

Self-assigning for review.

1. +++ b/core/modules/migrate/src/Plugin/migrate/destination/Config.php
   @@ -14,10 +14,52 @@
   + * Persist data to the config system. When a property is NULL, the default is
   

   Should be "Persists data..."

2. +++ b/core/modules/migrate/src/Plugin/migrate/destination/Config.php
   @@ -14,10 +14,52 @@
   + * Available configuration keys:
   + * - translations: (optional) Boolean, if TRUE, the destination is for
   + *   translations. Defaults to FALSE.
   

   The 'store null' option is not mentioned.

3. +++ b/core/modules/migrate/src/Plugin/migrate/destination/Config.php
   @@ -14,10 +14,52 @@
   + * This will add the value of the variable "node_admin_theme" to the config with
   + * the machine name "node.settings".
   

   This explanation is incomplete. It should mention that the saved value will really be node.settings:use_admin_theme (with nicer phrasing).

4. +++ b/core/modules/migrate/src/Plugin/migrate/destination/Config.php
   @@ -14,10 +14,52 @@
   + * This will add the value of the variable "site_offline_message" to the config
   + * with the machine name "system.maintenance" coupled with the relevant langcode
   + * as obtained from the "i18n_variable" source plugin.
   

   Again, this should be a little more explicit as to what the destination property will be.

5. +++ b/core/modules/migrate/src/Plugin/migrate/destination/Config.php
   @@ -14,10 +14,52 @@
   + * @see \Drupal\migrate_drupal\Plugin\migrate\source\d6\i18nVariable
   

   Nit: There should be a newline before this.

1. Corrected the typo.
2. Added the "store null" option (and removed its reference from the previous sentence to avoid repetition).
3. Included the name of the destination property.
4. Included the name of the destination property.
5. Added the newline.

Assigning to review this week.

I have reviewed the patch mentioned in comment #5, covered all the changes mentioned in #4.
Looks good.

1. +++ b/core/modules/migrate/src/Plugin/migrate/destination/Config.php
   @@ -14,10 +14,55 @@
   + * - store null: (optional) Boolean, if FALSE, the default is used when a
   

   What happens if the value is TRUE?

2. +++ b/core/modules/migrate/src/Plugin/migrate/destination/Config.php
   @@ -14,10 +14,55 @@
   + *   use_admin_theme: node_admin_theme
   ...
   + * the machine name "node.settings" as "node.settings.node_admin_theme".
   

   Wouldn't this become node.settings.use_admin_theme?

3. +++ b/core/modules/migrate/src/Plugin/migrate/destination/Config.php
   @@ -14,10 +14,55 @@
   + *   langcode: language
   + *   message: site_offline_message
   ...
   + * This will add the value of the variable "site_offline_message" to the config
   + * with the machine name "system.maintenance" as
   + * "system.maintenance.site_offline_message", coupled with the relevant langcode
   + * as obtained from the "i18n_variable" source plugin.
   

   Wouldn't this become:

   system.maintenance.langcode
   system.maintenance.message

Made all 3 corrections identified in #8.

I think this reads perfectly. It has my blessing. Let's git 'er done!

+++ b/core/modules/migrate/src/Plugin/migrate/destination/Config.php
@@ -14,10 +14,55 @@
+ * - translations: (optional) Boolean, if TRUE, the destination is for
+ *   translations. Defaults to FALSE.

Nit: We aren't being consistent. Over in #2862661-5: Add documentation to EntityConfigBase destination plugin.2 we don't like the wording for translations. Working on some fixes.

Committed/pushed to 8.4.x and cherry-picked to 8.3.x., thanks!