Doc fixes and refactorings based on comments from Eli Zaretskii

Doc fixes and refactorings based on comments from Eli Zaretskii

* doc/lispref/processes.texi (Network Processes): Clarify the
meaning of :tls-parameters.

* lisp/net/gnutls.el (open-gnutls-stream): Clarify :nowait.

* lisp/net/gnutls.el (gnutls-boot-parameters): Factor out into
own function.
(gnutls-negotiate): Use it.
(open-gnutls-stream): Ditto.

* src/eval.c (vformat_string): Refactor out the printing bits
from verror.
(verror): Use it.

* src/gnutls.c (boot_error): Mark failed processes with the
real error message.

* src/lisp.h: Declare vformat_string.