Document `descendant_count` and `family_count` properties of TagTreeModel

[jeffbowen]

Was about to build this myself before I realized that these properties already existed.

django-tagulous/tagulous/models/models.py

Lines 652 to 664 in 1617a62

	def _get_descendant_count(self):
	"The sum of the counts of all descendants"
	return self.get_descendants().aggregate(models.Sum("count"))["count__sum"] or 0
	descendant_count = property(
	_get_descendant_count, doc=_get_descendant_count.__doc__
	)
	def _get_family_count(self):
	"The count of self plus all its descendants"
	return self.descendant_count + self.count
	family_count = property(_get_family_count, doc=_get_family_count.__doc__)