doc: Add missing GUCs to SSL SNI docs

doc: Add missing GUCs to SSL SNI docs

The ssl_sni and hosts_file GUCs were missing from the configuration
section of the documentation, they were only described in the main
SSL SNI subsection.  This adds the GUCs to the relevant sections as
well as rewords the existing SSL SNI documentation to refer to the
settings along with a few smaller fixups.

Author: Daniel Gustafsson <daniel@yesql.se>
Reported-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwESD2Pty+J1kP3mXmWwMKZ5uJmknZdJsSGrMSRR6CQBmw@mail.gmail.com