Clarify what constitutes an event (bug#35238)

* doc/lispref/commands.texi (Input Events): Specify that events are
non-nil and remove vestiges of bug#10190.
* doc/lispref/os.texi (Recording Input): Document optional argument
of recent-keys.
* lisp/subr.el (eventp): Check that the car of conses is non-nil.
* etc/NEWS: Announce it as an incompatible change.
* src/keyboard.c (Frecent_keys): Clarify that returned "events" are
not real events.
1 job for master in 58 minutes and 58 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #1690
test-all

00:58:58

 
Name Stage Failure
failed
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
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:957: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1