taskgroup: unexport the Enter method of Throttle

The interesting part of the API is the Limit method.
Also unexport the Leaver type.

throttle.go

@@ -18,9 +18,9 @@ func NewThrottle(n int) Throttle {
 	return Throttle{adm: make(chan struct{}, n)}
 }
 
-// Enter blocks until a slot is available in t, then returns a [Leaver] that
+// enter blocks until a slot is available in t, then returns a [Leaver] that
 // the caller must execute to return the slot when it is no longer in use.
-func (t Throttle) Enter() Leaver {
+func (t Throttle) enter() leaver {
 	if t.adm == nil {
 		return func() {}
 	}
@@ -33,21 +33,21 @@ func (t Throttle) Enter() Leaver {
 	}
 }
 
-// A Leaver returns an in-use throttle slot to its underlying [Throttle].
+// A leaver returns an in-use throttle slot to its underlying [Throttle].
 // It is safe to call a Leaver multiple times; the slot will only be returned
 // once.
-type Leaver func()
+type leaver func()
 
 // Leave returns the slot to its [Throttle]. This is a legibility alias for
 // calling f.
-func (f Leaver) Leave() { f() }
+func (f leaver) Leave() { f() }
 
 // Limit returns a function that starts each [Task] passed to it in g,
 // respecting the rate limit imposed by t. Each call to Limit yields a fresh
 // start function, and all the functions returned share the capacity of t.
 func (t Throttle) Limit(g *Group) func(Task) {
 	return func(task Task) {
-		slot := t.Enter()
+		slot := t.enter()
 		g.Go(func() error {
 			defer slot.Leave()
 			return task()