Merge from origin/emacs-27

0a3682a566 * src/timefns.c: Add comments.
b16ba4041d ; lisp/emacs-lisp/seq.el: Explain why we don't use cl-lib ...
3cbf4cb796 Eliminate use of cl-concatenate in 'seq' package
363d927086 Fix bug with JIT stealth timers
818333c85a * doc/lispref/os.texi (time-subtract): Doc fix.

diff --cc doc/lispref/os.texi
Simple merge

diff --cc src/timefns.c
Simple merge