Feature summary: Have the description field in mw.guidedTour.TourBuilder's step support something similar to description: new mw.Title(...) that only retrieves tour description from a specific labeled section instead of a whole page.

Use case(s): Tours like the AELECT SecurePoll setup tour have to be kept in sync with the equivalent text-based notes, and being able to directly take the tour step description from the page would simplify a lot of things (currently, the tour is turned off due to the maintainability issues of having two parallel descriptions to keep in sync).

Other tours like the AfD guidance tour would also benefit from this, instead of having a proliferation of description pages that limits contributions to the more technically minded.

Currently, something like description: new mw.Title('User:Chaotic Enby/sandbox#Section') doesn't work, and returns the entire page as description instead.

Benefits: Currently, complex guided tours have the choice between storing text in the tour code itself with mw.guidedTour.WikitextDescription (which is problematic for maintainability purposes, as it can only be edited by interface administrators) and storing each step's text on a separate page with mw.Title. However, this can also quickly become overwhelming for tours with more than a few steps, especially if we want to keep them in sync with other pages.

Being able to have all of the tour's descriptions stored on a single page would help alleviate this maintenance burden, especially if we want to keep the tour in sync with a text-only documentation.