fix golint issues in core/mathx (#498)

core/mathx/proba.go

@@ -6,18 +6,21 @@ import (
 	"time"
 )
 
+// A Proba is used to test if true on given probability.
 type Proba struct {
 	// rand.New(...) returns a non thread safe object
 	r    *rand.Rand
 	lock sync.Mutex
 }
 
+// NewProba returns a Proba.
 func NewProba() *Proba {
 	return &Proba{
 		r: rand.New(rand.NewSource(time.Now().UnixNano())),
 	}
 }
 
+// TrueOnProba checks if true on given probability.
 func (p *Proba) TrueOnProba(proba float64) (truth bool) {
 	p.lock.Lock()
 	truth = p.r.Float64() < proba