Add documentation to UrlEncode process plugin

Self-assigning for review.

+++ b/core/modules/migrate/src/Plugin/migrate/process/UrlEncode.php
@@ -9,10 +9,20 @@
+ * Example:

If the source URL is "http://example.com/this is a url with spaces.html", this process plugin will return "http://example.com/this%20is%20a%20url%20with%20spaces.html"

Add more explanation to the example, as suggested by @ultimike.

1. +++ b/core/modules/migrate/src/Plugin/migrate/process/UrlEncode.php
   @@ -9,10 +9,24 @@
   + * Applies urlencoding to the source value.
   

   'urlencoding' is not a word. Also, kinda passive-voice. Maybe we could rephrase this as "URL-encodes the input value"?

2. +++ b/core/modules/migrate/src/Plugin/migrate/process/UrlEncode.php
   @@ -9,10 +9,24 @@
   + * The urlencode plugin is needed when the URI is to be opened by a later
   + * migration stage, and the source URI value is not already encoded.
   

   I think we could strike this entire sentence. It does not explain anything about the plugin itself, just about how it interacts with a migration...under certain circumstances.

3. +++ b/core/modules/migrate/src/Plugin/migrate/process/UrlEncode.php
   @@ -9,10 +9,24 @@
   + * This will convert the source url 'http://example.com/a url with spaces.html'
   

   s/url/URL.

4. +++ b/core/modules/migrate/src/Plugin/migrate/process/UrlEncode.php
   @@ -21,7 +35,22 @@
   -   * {@inheritdoc}
   +   * Performs the associated process.
   +   *
   +   * @param string $value
   +   *   The input URL
   +   * @param \Drupal\migrate\MigrateExecutableInterface $migrate_executable
   +   *   The migration in which this process is being executed.
   +   * @param \Drupal\migrate\Row $row
   +   *   The row from the source to process.
   +   * @param string $destination_property
   +   *   The destination property currently worked on. This is only used together
   +   *   with the $row above.
   +   *
   +   * @return string
   +   *   The sub string of $value.
   +   *
   +   * @throws \Drupal\migrate\MigrateException
   

   This should be {@inheritdoc}.

Changes in response to the code review by @phenaproxima in #10.

Looks good to me.

Short and sweet. Thanks, all! Committed to 8.4.x, and backported to 8.3.x as a documentation improvement.