Add documentation to Entity destination plugin

This one is a bit tricky, but here's a start. I have added:

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

I couldn't identify destination properties the same as I did with #2862655: Add documentation to Config destination plugin and #2862751: Add documentation to PerComponentEntityDisplay destination plugin.

Self-assigning for review.

Good start!

1. +++ b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
   @@ -13,6 +13,48 @@
     * Provides entity destination plugin.
   

   Can this say "Provides a generic destination to import entities", or something else a little more illuminating?

2. +++ b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
   @@ -13,6 +13,48 @@
   + * Available configuration keys:
   + * - translations: (optional) Boolean, if TRUE, the destination is for
   + *   translations. Defaults to FALSE.
   

   I think we'll need to expand quite a bit on this. Also, what happens if the entity type being imported does not support translations?

3. +++ b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
   @@ -13,6 +13,48 @@
   + * This will add the results of the process plugin (for example "nid", "vid" and
   + * "title") to a "node" entity.
   

   I'd rather rephrase this to "This will save the processed, migrated row as a node."

4. +++ b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
   @@ -13,6 +13,48 @@
   + * This will add the results of the process plugin (for example "nid", "vid" and
   + * "title") to a "node" entity associated with the relevant langcode because the
   + * translations configuration is set to "true".
   

   I'd like to rephrase this similarly.

Changes made as per comment #5.

Thanks, @Yogesh Pawar!

+++ b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
@@ -33,8 +33,8 @@
+ * This will save the processed, migrated row as a node (for example "nid",
+ * "vid" and "title") to a "node" entity.

This phrasing is weird -- it'll save to a node as a node entity? I think we need to streamline this.

I have to leave this NW because not all my feedback from #5 has been addressed.

Addressed @phenaproxima's remaining feedback from #5.

+++ b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
@@ -11,7 +11,48 @@
+ *   plugin: entity:node

Do we want to document anything about the entity:node plugin argument? How it auto finds the generic entity plugin and uses the second part as the entity_type?

Personally, I don't think we need to. There's nothing Migrate-specific about that; it's how derived plugins work. Besides, I think that to Migrate's mainly developer audience, it will be pretty self-explanatory.

Committed as 2225b0a and pushed to 8.5.x.