Add documentation to Concat process plugin

Self-assigning for review.

1. +++ b/core/modules/migrate/src/Plugin/migrate/process/Concat.php
   @@ -8,9 +8,43 @@
   + * The concat process plugin is used to concatenate strings in the source. An
   

   Can we say "The concat process plugin is used to concatenate a set of strings", and drop "in the source"?

2. +++ b/core/modules/migrate/src/Plugin/migrate/process/Concat.php
   @@ -8,9 +8,43 @@
   + * example use case would be when the source data contains an array of strings
   + * that should be imploded into a single value.
   

   Let's change this to "An example use case would be imploding a set of strings into a single value."

3. +++ b/core/modules/migrate/src/Plugin/migrate/process/Concat.php
   @@ -8,9 +8,43 @@
   + * Available configuration keys
   

   Needs to end with a colon.

4. +++ b/core/modules/migrate/src/Plugin/migrate/process/Concat.php
   @@ -8,9 +8,43 @@
   + * This will set new_text_field to foobar.
   

   This is not accurate. It will set new_text_field to the concatenation of the 'foo' and 'bar' source values. For example, if the 'foo' property is "wambooli" and the 'bar' property is "pastafazoul", new_text_field will be "wamboolipastafazoul". Let's use a concrete example like that here.

5. +++ b/core/modules/migrate/src/Plugin/migrate/process/Concat.php
   @@ -8,9 +8,43 @@
   + * This will set new_text_field to foo/bar.
   

   Let's update this example for accuracy as well.

6. +++ b/core/modules/migrate/src/Plugin/migrate/process/Concat.php
   @@ -20,9 +54,22 @@
   +   * Concatenates the strings in the source value.
   +   *
   +   * @param array $value
   +   *   The input array.
   +   * @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 imploded value.
       *
   -   * Concatenates the strings in the current value.
   +   * @throws \Drupal\migrate\MigrateException
   

   This should use {@inheritdoc}.

I'll pick this up and incorporate the changes phenaproxima suggests

Here's the edits.

This is really close. Consider these nitpicks...

1. +++ b/core/modules/migrate/src/Plugin/migrate/process/Concat.php
   @@ -8,9 +8,47 @@
   + * Concatenates the strings in the source.
   

   I'm not a huge fan of "in the source". Can we simply say "Concatenates a set of strings"?

2. +++ b/core/modules/migrate/src/Plugin/migrate/process/Concat.php
   @@ -8,9 +8,47 @@
   + * The concat process plugin is used to concatenate strings.
   + * An example use case would be imploding a set of strings into a single value.
   

   Let's clean this up a little: "The concat plugin is used to concatenate strings. For example, imploding a set of strings into a single string."

3. +++ b/core/modules/migrate/src/Plugin/migrate/process/Concat.php
   @@ -8,9 +8,47 @@
   + * property is "pastafazoul", new_text_field will be "wamboolipastafazoul".
   

   I smile upon pastafazoul :)

Works for me.

+++ b/core/modules/migrate/src/Plugin/migrate/process/Concat.php
@@ -8,9 +8,47 @@
+ * You can also specify a delimiter.

In implode() and explode() terms, this should be called "glue", not "delimiter".

I like that. I am personally in favor of "delimiter", even though the PHP documentation uses "glue". I think the word "glue" makes huge sense in the context of explaining the implode() function, but less when explaining a process plugin that wraps around it. Therefore, this is a good compromise. Combined with the excellent examples, this documentation is now crystal-clear. And I'm so pleased to be, as far as I know, the person who originally incepted the term "pastafazoul" into core.

We had to google "wambooli" to determine if it is an offensive word. It seems not. Pro tip: new nonsense words in core require more committer time ;-).

Committed 48ec916 and pushed to 8.4.x. As an API documentation improvement, it also goes in 8.3.x. Thanks!

Wambooli is a nonsense word I picked up from C for Dummies, the best programming book ever written. ;-)

We're getting there...