Improve recent doc fix for cl-reduce

* lisp/emacs-lisp/cl-seq.el: (cl-reduce): Clarify treatment of
:INITIAL-VALUE when :FROM-END is non-nil.  Improve wording when SEQ
is empty and :INITIAL-VALUE is unspecified (bug#37400).  Describe
behavior when SEQ comprises a single element.
1 job for master in 90 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
failed #3658


Name Stage Failure
test-all Test The script exceeded the maximum execution time set for the job
make -C test check-expensive
make[1]: Entering directory '/builds/emacs/emacs/test'
rm -f ./*.tmp
make[2]: Entering directory '/builds/emacs/emacs/test'
make[3]: Entering directory '/builds/emacs/emacs/test'
ELC lisp/net/tramp-tests.elc
GEN lisp/net/tramp-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds