doc/guide: Reword CONFLICTS usage suggestions

Despite most CONFLICTS are automatically detected at package
installation time it is still a good idea to manually mark CONFLICTS
in order to fail as early as possible instead e.g. of failing at
package installation time after possibly installing several (maybe
not needed) dependencies.

Pointed out by discussion on pkgsrc-changes@ by <jperkin>:

<https://mail-index.NetBSD.org/pkgsrc-changes/2018/09/09/msg180626.html>

Last two paragraphs from <jperkin>, thanks!

Also discussed with <wiz>, thanks!

(leot)