Improve documentation around standard error pipes (Bug#35328).

* doc/lispref/processes.texi (Asynchronous Processes): Document
existence and properties of the standard error process.
(Accepting Output): Document that one has to accept output from the
standard error process separately.
1 job for master in 51 minutes and 58 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #1476
test-all

00:51:58