Be more specific in resolved.conf man page with regard to DNSOverTLS

Be more specific in resolved.conf man page with regard to DNSOverTLS

DNSOverTLS in strict mode (value yes) does check the server, as it is said in
the first few lines of the option documentation. The check is not performed in
"opportunistic" mode, however, as that is allowed by RFC 7858, section "4.1.
Opportunistic Privacy Profile".

> With such a discovered DNS server, the client might or might not validate the
> resolver. These choices maximize availability and performance, but they leave
> the client vulnerable to on-path attacks that remove privacy.