Improve and clarify documentation of subprocesses

* doc/lispref/processes.texi (Subprocess Creation, Shell Arguments):
Mention 'make-process' rather than 'start-process'.  Update wrt
standard destinations of standard output/error streams and due to
different formats of arguments accepted by 'make-process'.
(Processes): Mention process objects that represent connections.
(Synchronous Processes): Minor clarifications.
(Asynchronous Processes): Describe 'make-process' and
'make-pipe-process' before 'start-process'.  Update and expand the
documentation.
(Deleting Processes, Process Information, Input to Processes)
(Signals to Processes, Query Before Exit, Network): Update and
expand the documentation, especially wrt process objects that
represent connections.
(Output from Processes): Mention the possibility of separating
stderr via 'make-process'.
(Filter Functions): Mention that stderr by default arrives at the
filter function together with stdout.  (Bug#24287)

* src/process.c (Fprocess_id, Fprocess_command)
(Fprocess_contact, Fprocess_type, Fstop_process): Doc fixes for
process objects that represent connections.