Improve API documentation of DrupalSqlBase source plugin

A hole in one! This looks fantastic to me.

However, I'd like to postpone it on #2862671: Add documentation to SqlBase source plugin, since that contains important documentation that is very relevant to DrupalSqlBase. Once that one lands, I think this can go direct to RTBC.

+++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/DrupalSqlBase.php
+++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/DrupalSqlBase.php
@@ -13,10 +13,17 @@

@@ -13,10 +13,17 @@
+ * For available configuration keys, please refer to the parent classes

Just a note that our style guidelines indicate that the word "please" should be omitted. Reference: https://www.drupal.org/node/604342 (See also: #679890: Using "Please" in the interface).

Minor stuff. As always.

1. +++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/DrupalSqlBase.php
   @@ -13,10 +13,17 @@
   + * when writing source plugins that use a Drupal database a source, for example:
   

   Should be "...use a Drupal database as a source..." (missing 'as')

2. +++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/DrupalSqlBase.php
   @@ -13,10 +13,17 @@
   + * - For full list, please refer to the methods of this class.
   

   This should be its own paragraph, not a bullet point.

3. +++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/DrupalSqlBase.php
   @@ -13,10 +13,17 @@
   + * For available configuration keys, refer to the parent classes
   

   Need to have a colon after "classes". Also, maybe the SqlBase and SourcePluginBase references should have @see annotations?

Addressed nits from #8.

Looks good to me.

1. +++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/DrupalSqlBase.php
   @@ -13,10 +13,19 @@
   + * - Check if given module exists in the source database.
   

   Nit: s/if given/if the given/

2. +++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/DrupalSqlBase.php
   @@ -13,10 +13,19 @@
   + * For full list, please refer to the methods of this class.
   

   Nit: s/For full/For a full/

Addressed nits from #13.

Thanks, Jo. Back to RTBC!

+++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/DrupalSqlBase.php
@@ -13,10 +13,19 @@
+ * This class provides general purpose helper methods that are commonly needed

Can this just be 'Provides...'?

Otherwise looks good.

Made the change requested in #16.

Great, everything addressed.

+++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/DrupalSqlBase.php
@@ -52,7 +61,7 @@ public function __construct(array $configuration, $plugin_id, $plugin_definition
+   * Retrieves all system data information from the source Drupal database.

Personally, I really like the consistent use of 'the source Drupal database' throughout. It leaves no ambiguity at all about the source.

Committed/pushed to 8.5.x and cherry-picked to 8.4.x. Thanks!