Sadly, some brilliant text had to be jettisoned from the Definitive Guide. Some of it lives here.
Castaways from the writing process:
Of course we should have an API or three. We know Jeff Eaton would stick APIs in a novelty module called Eldorado Superfly. No, actually, he didn't. From this we can learn that if our module is simple enough and uses enough of Drupal's APIs, it doesn't need to create its own.
[Eaton] drawing on his experience with VotingAPI, which, we might note, puts API right in the name of the module, and if that doesn't mean something, what does?
Drupal is going about its business, which usually means displaying a web page. It asks the theme what regions it has available. "About a bazillion," says Bartik. "Well," says Drupal, "I'm going to admin/structure so that uses the administration theme, Seven." Bartik leaves in a huff. Seven says, "here are my regions." Drupal figures out what blocks are are in each region.
system_block_view has a switch statement on this parameter, for the help case it calls the function menu_get_active_help(), which lives in menu.inc, a part of Drupal's base system that every module has access to.
This one turned out to be really a case of a Local Task (tab) that should not have any description– they don't get displayed, not even as title tip text.
Note The lack of a period on the description "Administer content" appears to be an unintentional violation of Drupal's coding standards. Even the vigilant contributors can Drupal core can occasionally miss the smallest details, but rest assured that this will get fixed in Drupal 8!