What kind of content ends up in the "System help" block? How could I answer this question myself?
Help content, defined in hook_help is added to the "System help" block for display in

system_block_view

via

menu_get_active_help

which returns the valid help content for the current menu path.


  How could I answer this question myself?


You can answer this question yourself by


Studying the Drupal API
Grepping / searching the codebase for relevant text
Asking on Drupal Answers ;)