defaults: add common explanation of annotations

We don't explain what the annotations in mean in the defaults file.  This
has led to confusion with people misunderstanding the annotation and
mistakenly copying the annotation with configuring dCache.

This patch adds a common information about what the annotations mean.  It
also refactors the common explanation so the current duplication is reduced
(but not eliminated).

Target: master
Ticket: http://rt.dcache.org/Ticket/Display.html?id=8166
Request: 2.7
Request: 2.6
Request: 2.2