doc: explain NoPos.IsValid() with parentheses.

[iporsut]

I found in "go/token" explain NoPos call method IsValid() with parentheses. I think it should no parentheses NoPos.IsValid().

"go/token/position.go:74"

// associated with it, and NoPos().IsValid() is false. NoPos is always

[gopherbot]

CL https://golang.org/cl/42992 mentions this issue.