bug#47494: 28.0.50; describe-mode is now slow

[Mark Oteiza]

>From -Q: C-h m

Takes about a second here.  Looking at the profiler, it's split between
some new function and the garbage collector:

         118  52% - #<compiled 0x97675bce2c2c377>
         118  52%  - mapatoms
         118  52%   + help-fns--list-local-commands
          92  41%   Automatic GC
          <snip>

https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=98e3ee27472


In GNU Emacs 28.0.50 (build 2, x86_64-pc-linux-gnu, X toolkit, cairo version 
1.17.4, Xaw scroll bars)
 of 2021-03-30 built on logos
Repository revision: 4bfcfdb41638f37c89c0db8f297707773c677656