More process-related doc and manual small edits

More process-related doc and manual small edits

* doc/lispref/processes.texi (Output from Processes, Filter Functions):
Mention waiting-for-user-input-p.
(Sentinels, Query Before Exit, System Processes, Transaction Queues):
(Network Servers, Datagrams, Network Processes, Network Options)
(Network Feature Testing, Serial Ports): Copyedits.
(Network): Add encrypted network overview paragraph.
Cross-reference the Emacs-GnuTLS manual.  Use @acronym.

* lisp/net/network-stream.el (open-network-stream): Doc fix.

* src/process.c (Fset_process_inherit_coding_system_flag)
(Fset_process_query_on_exit_flag): Doc fix (mention return value).
(Fmake_network_process): Doc fix.