security/py-{acme,certbot}: Update to 0.18.2

certbot produces an error [1] when used with versions of pyOpenSSL > 17.2.0,
who's port was recently updated [2] to 17.3.0.

This version update contains a fix for that issue.

While I'm here, improve pkg-message usage invocation example and provide
a link to documentation

[1] https://github.com/certbot/certbot/issues/5111
[2] http://svnweb.freebsd.org/changeset/ports/450350

Reported by: Daniel Boothby (via private email)
Approved by: cpm (py-acme maintainer)