fix docs for filter_by arguments

It seems that ``base`` arg is only on Lithium (develop branch)
and the ``default`` arg was actually added on 2014.1.0.
This is particularly confusing because its usage is shown on formulas docs
as a best practice

http://docs.saltstack.com/en/latest/topics/development/conventions/formulas.html#collecting-common-values
Merge pull request #18111 from slafs/grains-filterby-docfix

fix docs for filter_by arguments