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: NErik Skultety <eskultet@redhat.com>

src/remote/libvirtd.conf

@@ -366,7 +366,9 @@
 #    4: ERROR
 #
 # Multiple filters can be defined in a single @filters, they just need to be
-# separated by spaces.
+# separated by spaces. Note that libvirt performs "first" match, i.e. if
+# there are concurrent filters, the first one that matches will be applied,
+# given the order in log_filters.
 #
 # e.g. to only get warning or errors from the remote layer and only errors
 # from the event layer: