math: document special-cases behavior for Dim, Max and Min

math: document special-cases behavior for Dim, Max and Min

Max returns +Inf if x or y is +Inf; else it returns NaN if either x or y is NaN. Max(-0, -0) returns -0.
Min returns -Inf if x or y is -Inf; else it returns NaN if either x or y is NaN. Min(+0, -0) returns -0.
Dim(+Inf, +Inf) = NaN, Dim(-Inf, -Inf) = NaN and Dim(NaN, anything) = NaN.
Also, change "conditions" to "cases" for Sin (missed it in previous CL).

R=rsc, dave
CC=golang-dev
https://golang.org/cl/5437137