Add documentation to Route process plugin

Self-assigning for review.

1. +++ b/core/modules/migrate/src/Plugin/migrate/process/Route.php
   @@ -11,10 +11,27 @@
   - * @link https://www.drupal.org/node/2750777 Online handbook documentation for route process plugin @endlink
   + * The route process plugin ...
   

   This seems incomplete :)

2. +++ b/core/modules/migrate/src/Plugin/migrate/process/Route.php
   @@ -11,10 +11,27 @@
   + * The source value is an array of two values
   + *   - link_path:
   + *   - options:
   

   The first sentence needs to end with a colon, and both items in the input value need to be explained.

3. +++ b/core/modules/migrate/src/Plugin/migrate/process/Route.php
   @@ -11,10 +11,27 @@
   + * @code
   + * process:
   + *   route:
   + *     source:
   + *       - link_path
   + *       - options
   + * @endcode
   

   We need an explanation of what this example does. And maybe another example as well, if it would be helpful.

Changes in response to code review:

1. Completed the sentence.
2. Added the colon and explained the values.
3. Added and explained two examples.

Self-assigning for review.

1. +++ b/core/modules/migrate/src/Plugin/migrate/process/Route.php
   @@ -11,10 +11,50 @@
   + * The route process plugin sets the destination route information based on the
   + * source link_path.
   

   This effectively repeats the short description. We're going to need to either explain this a lot more thoroughly (i.e., by linking to the main documentation for the D8 routing system) or remove it altogether.

2. +++ b/core/modules/migrate/src/Plugin/migrate/process/Route.php
   @@ -11,10 +11,50 @@
   + * process:
   + *   new_route_field:
   + *     plugin: route
   + *     source:
   + *       - 'https://www.drupal.org'
   + *       -
   + *         attributes:
   + *           title: Drupal
   + * @endcode
   + *
   + * This will set new_route_field to be a route with the URL
   + * "https://www.drupal.org" and title attribute "Drupal".
   

   Can we add an example showing what will be returned if you pass an internal path like /admin/content as the source value?

1. Removed the repeated line.
2. Corrected the second example which should have already referenced an internal path (at least that's what I'd put in the comment).

I like it.

Committed and pushed 2cfd4d7 to 8.4.x and 63ab9aa to 8.3.x. Thanks!