Docs: Fixed bad exists? documentation.

First I want to say, I am not sure of the best way of fixing 2.3 docs, since docrails only has the one branch. If this isn't the right way I can ammend this. Thanks.

Base#exists? does not actually take options like finder methods. Trying
to use what the documentation suggests will return a PG error because it
will look for a column named 'conditions'.

I changed the documentation to reflect how the exists? method actually
works in 2-3.