Add comment to eieio-opt about why we're requiring cl-extra

* lisp/emacs-lisp/eieio-opt.el (cl-extra): Add comment about why
we're not requiring cl-lib.
1 job for master in 54 minutes and 10 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #2232


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