Allow open-network-stream to use different TLS capability commands

Allow open-network-stream to use different TLS capability commands

* doc/lispref/processes.texi (Network): Document non-string
capability command.
* lisp/gnus/nntp.el (nntp-open-connection): Use HELP for Typhoon
and CAPABILITIES for everything else (bug#41960).

* lisp/net/network-stream.el (open-network-stream): Document
function variety of :capability-command.
(network-stream-open-starttls): Use it.
(network-stream-open-tls): Ditto.
(network-stream-open-shell): Ditto.
(network-stream--capability-command): New helper function.