math/big: Rat.FloatString does not document what rounding method it uses

[tv42]

This came up while somebody was talking about using it for currency math.

https://en.wikipedia.org/wiki/Rounding#Tie-breaking

[griesemer]

It's rounding to nearest even but it should be documented.

[griesemer]

This was fixed a while back. The (now deleted) additional CL was incorrectly referring to this issue.