added NaN doc for trunc
added NaN doc for trunc

stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/stan