Backport doc improvement in ELisp manual

* doc/lispref/processes.texi (Accepting Output): Backport:
document how do avoid race conditions while waiting for all of
the process's output to arrive.
1 job for emacs-26 in 26 minutes and 18 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #1445
test

00:26:18