[Meta] Add source plugin documentation to the codebase

This issue is to track progress on adding documentation to all source plugins.

This is suitable for a novice.

How to proceed.

1. Search the list of source plugins and select a group of source plugins to work on. Priority is given to any group that has a source plugin with a '*' after the file name. Those are the one that use configuration properties.
2. Create a child issue
3. For the issue summary
4. In the Problem/Motivation section use this text, "This issue is to add documentation for the following source plugins." And then add the list of source plugins.
5. In the 'Remaining Tasks' section Add what is still left to do, For example, this could be a list of the source plugins that still need documentation.
6. For the document
7. Use the same style and formatting as was done for the process plugins in the migrate module.
8. See #3189587: Add documentation for taxonomy terms source plugins for an example
9. Include an * @see \Drupal\migrate\Plugin\migrate\source\SqlBase which has more configuration options.

Refer to API documentation and comment standards for the order of the sections.

If you have any questions, eith leave a comment here or ask in #migration on Drupal Slack.

In progress

Taxonomy and Node - #3209353: Add documentation for remaining node and taxonomy modules

• d6/VocabularyPerType.php
• d6/Vocabulary.php
• d6/VocabularyTranslation.php
• d7/Vocabulary.php
• d7/VocabularyTranslation.php *
• d6/NodeComplete.php
• d6/NodeRevision.php
• d6/NodeType.php
• d7/NodeComplete.php
• d7/NodeRevision.php
• d7/NodeType.php
• NodeCounter.php
• d6/TermNode.php
• d6/TermNodeRevision.php

Remaining items - #3199741: Add documentation for remaining source plugins

• Aggregator
  • AggregatorFeed.php
  • AggregatorItem.php
• Url
  • d6/UrlAlias.php
  • d7/UrlAlias.php
  • UrlAliasBase.php
• Tracker
  • d7/TrackerNode.php
  • d7/TrackerUser.php
• Shortcut
  • d7/Shortcut.php
  • d7/ShortcutSet.php
  • d7/ShortcutSetUsers.php
• Search
  • d6/SearchPage.php
  • d7/SearchPage.php
• Role
  • d6/Role.php
  • d7/Role.php
• Profile
  • d6/ProfileFieldOptionTranslation.php
  • d6/ProfileFieldTranslation.php
  • d6/ProfileFieldValues.php
• Menu
  • MenuLink.php
  • Menu.php
  • d6/MenuLinkTranslation.php
  • d7/MenuLinkLocalized.php
  • d7/MenuLinkTranslation.php
  • d7/MenuTranslation.php
• Image
  • d6/ImageCachePreset.php
  • d7/ImageStyles.php
  • d7/ResponsiveImageStyles.php
• Filter
  • d6/FilterFormat.php
  • d7/FilterFormat.php
• ViewMode
  • d6/ViewModeBase.php
  • d6/ViewMode.php *
  • d7/ViewMode.php
• Contact
  • ContactCategory.php
  • ContactSettings.php
• Color/Theme
  • d7/Color.php
  • d7/ThemeSettings.php
• Comment
  • CommentType.php
  • d6/Comment.php
  • d7/CommentEntityTranslation.php
  • d7/Comment.php
• • Action.php
  • Extension.php *
  • I18nQueryTrait.php
  • UpdateSettings.php
  • d7/BlockedIps.php
  • d7/RdfMapping.php
  • Book.php
  • d7/EntityTranslationSettings.php

File - #3189876: Add documentation for file source plugins

• d6/UploadInstance.php
• d6/Upload.php
• d6/File.php *
• d7/File.php *

Done
Migrate and Migrate Drupal - #3206939: Add documentation for Migrate and Migrate Drupal source plugins
Field (d6) - #3200809: Add documentation for d6 field source plugins
Block - #3199742: Add documentation for block source plugins
Language - #3199999: Add documentation for language source plugins
Node - #2937989: Support multiple node types in node source plugins
Taxonomy term - #3189587: Add documentation for taxonomy terms source plugins
User - #3200735: Add documentation for user, profile and roles source plugins
Variable - #3196177: Add documentation for variable source plugins
Field (d7) - #3204986: Add documentation for d7 field source plugins

• DrupalSqlBase.php