Update documentation for AxisGridlines

Adds docs for lineColor and lineWidth, and updated docs for allGridlines
Removed comments for drawGridlines, so docs are not generated.
Fixes up Axis documentation

Removes documentation from private methods, and adds it to some public
methods which were missing it.

Also renames series.accessor as "valueFunction", and adds a findMin
method to the axis. Removes the vertical() method from the axis, as this
was just !horizontal().