As reported by Eloy Lafuente (stronk7):
The watch/unwatch behavior is not comfortable. And it's in fact way better in other skins like Vector.
Right now it moves to a "you are now watching this page" page... and need to click again to go back to the page. I think that was automatic before, with some info showing automatically or so.
While viewing a page
Link "Watch" (or in Vector skin the white star), located at the margin of each page, adds the current page to your watchlist, and turns the link into "Unwatch" (or in Vector skin a blue star).
The message with id 'addedwatchtext' (talk) or message with id 'removedwatchtext' (talk), respectively, is shown above the page, and "Watch" is changed to Unwatch or conversely. Depending on the browser this is done without reloading the whole page, using AJAX with ajaxwatch.js.
The old Moodle docs theme allowed you to watch and unwatch pages without reloading the whole page, whereas the new docs theme does not.