porters-handbook: clarify semantics of CONFLICTS* definitions

There has been some confusion about the correct specification of
CONFLICTS definitions in port Makefiles.

The examples hint at the correct use of the package base name when
possible (i.e. no version check required), but there were quite a
few definitions that included unneccessary wild-card patterns to
match all versions.

Since I had to explain the semantics by mail to several maintainers
after fixing up stale or incorrect definitions and since I received
a request to clarify the semantics of CONFLICTS definitions, I have
created a new section that explains the correct use.

After having the change available for review for 2 months without
any reaction of the assigned reviewers and since the next round of
CONFLICTS fixes is likely to result in many requests to explain
the correctness of the changes performed, I'm committing this text
now.

Differential Revision: https://reviews.freebsd.org/D34352