fix docs for filter_by arguments

It seems that ``base`` arg (introduced by Thayne Harbaugh) is only on Lithium (develop branch)
and the ``default`` arg was actually added on 2014.1.0.
This is particularly confusing because usage of ``base`` arg is shown on formulas docs
as a best practice

http://docs.saltstack.com/en/latest/topics/development/conventions/formulas.html#collecting-common-values

As I side note I'm really sad that ``base`` arg didn't make it to Helium release :/