We have added a new search type (that is, a new @SearchPlugin plugin) to Core, to go along with the Node and User search types that already existed. This search type is for searching help, and its plugin is called \Drupal\help_topics\Plugin\Search\HelpSearch (it is in the experimental Help Topics module). Using this type of search, administrators can define search pages that will provide help search to site users with permission to view help.
Modules that provide @HelpSection plugins (which define sections for the admin/help page) can make their help text searchable by implementing a new interface \Drupal\help_topics\SearchableHelpInterface on their HelpSection class, which requires implementing two methods: listSearchableTopics() to list the IDs of help topics that should be searchable, and renderTopicForSearch(), which renders a single ID for either help indexing or help search results. As an example, see the \Drupal\help_topics\Plugin\HelpSection\HelpTopicsSection plugin, which is currently the only HelpSection plugin that implements this interface. (So, currently if you search help, you are only searching Help Topics managed by the Help Topics experimental module.)
Note that there are plans to merge the Help Topics experimental module into the existing core Help module when it gets to be time for a stable release. So the namespaces above containing "\Drupal\help_topics" are most likely going to change to "\Drupal\help" instead.
