Problem/Motivation
The help system needs search capability.
Proposed resolution
Probably, using the existing Drupal core Search module to build the search functionality would make the most sense.
Remaining tasks
a) Implement search indexing and searching, using the core Search module. See comment #5 in this issue for some notes on how to do that.
b) Add a search box to the main Help page, and possibly to each individual help topic page.
c) Consider adding a keywords field to help topics. Entered keywords could be emphasized in the search index (the same way as the title and h2/h3/etc. headings on the page are emphasized), so that if a user searches on one of the keywords, they would be more likely to get that page in the results.
User interface changes
Help will be searchable.
API changes
No.
Data model changes
Might add a keywords field to help topics.
Original summary
It seems like a search feature or an index would be useful for the help topics system.
Search: could use core Search and index each topic's full text. It could also include hook_help() topics. The search could appear on the admin/help page as a "section".
Index: Could have a way to add index entries to each topic, and there could be an automatically-generated index that would be built during a cron job, which would list the index entries with links to the topics.
So this issue:
a) Decide on what feature(s) to add.
b) Build them.