Make delimit-columns-region say what it does

* lisp/delim-col.el (delimit-columns-region): Make the doc string
say a bit more about what it does (bug#35651).
(delimit-columns-rectangle): Refer to the first function.
1 job for master in 55 minutes and 41 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
failed #2443
test-all

00:55:41

 
Name Stage Failure
failed
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:318: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1