Improve 'open-network-stream' documentation.

Improve 'open-network-stream' documentation.

* doc/lispref/processes.texi (Network): Correct explanation of
':warn-unless-encrypted'.  Document ':error' return keyword.
* lisp/net/network-stream.el (open-network-stream): Improve
':return-list' documentation.  Document ':error'.  Correct
explanation of ':warn-unless-encrypted'.