Bear in mind that docstrings are compiled into the JAR that ships as "Clojure" so increasing the docstrings has a (small) direct cost for everyone using Clojure, regardless of whether they look at the docstrings.
Historically, the docstrings have always been a short, concise description of the behavior that is expected to be augmented in guides/reference material on clojure.org and the various community-managed documentation sites (or in books).
There are a LOT of clojure.core docstrings that could justifiably be enhanced, either to provide clarifications such as you highlight here, or to add examples, or expand the description, or more clearly define terms used in them -- but all of that adds overhead to the Clojure artifact so none of it should be undertaken lightly. Changes to Clojure itself -- even just docstring changes -- require work from Clojure's core maintenance team at Cognitect so it takes away their time from other things that benefit users of the language.
Expanding the material on clojure.org itself has no such cost (other than the willingness of volunteers to contribute their time to create/expand that material) -- beyond the moderation effort of Alex et al to review and merge such content, which bar is a lot lower than for Clojure itself.