libvirtd.conf: Document that we do a 'first' match on log filters

libvirtd.conf: Document that we do a 'first' match on log filters

When applying the log filters, one has to define the more specific
filters before the generic ones, because the first filter that matches
will be applied. However, we've been missing this information in the
config, so it always has been a trial-error scenario figuring out that
e.g. '4:util 1:util.pci' doesn't actually enable verbose logging on the
src/util/virpci.c module because 4:util will be matched first.

Signed-off-by: Erik Skultety <eskultet@redhat.com>