Add documentation to PerComponentEntityDisplay destination plugin

Added the configuration keys and an example.

Self-assigning for review.

1. +++ b/core/modules/migrate/src/Plugin/migrate/destination/PerComponentEntityDisplay.php
   @@ -5,6 +5,44 @@
   + * Available configuration keys:
   + * - entity_type: The entity type.
   + * - bundle: The entity bundle.
   + * - view_mode: The view mode.
   + * - field_name: The name of the field.
   + * - options: The display options.
   

   These aren't configuration keys, they're the destination properties expected in the imported row. I think we'll need to rephrase this again. Also, the property descriptions are a bit threadbare. It's not clear, for example, if entity_type is supposed to be a string. I'd suggest the following:

   entity_type: The entity type ID.
   bundle: The entity bundle.
   view_mode: The machine name of the view mode.
   field_name: The machine name of the field to be imported into the display.
   options: (optional) An array of options for displaying the field in this view mode.

2. +++ b/core/modules/migrate/src/Plugin/migrate/destination/PerComponentEntityDisplay.php
   @@ -5,6 +5,44 @@
   + * This will create a "user_picture" image field on the "default" view mode of
   

   It's misleading to say that a field is created. How about "This will add the user_picture image field to the default view mode of user entities, with options defined by the 'options' constant (i.e., the label will be hidden, no image style will be applied, etc.)"

Made the changes suggested by @phenaproxima in #4.

Looks terrific!

Superb, thanks, looks good, committed.