[openbsd-pkg] Add note regarding package flavors

This adds a note to the openbsd-pkg module which explains how to specify
package flavors as well as how to avoid ambiguity errors on a package
with multiple flavors.

Technically, this is not Ansible-specific, but it's pretty likely the
user will encounter this problem.
Merge pull request #7774 from mwpher/devel

[openbsd-pkg] Add note regarding package flavors