add some comments

Signed-off-by: zyguan <[email protected]>

internal/locate/region_cache.go

@@ -551,6 +551,7 @@ func (r *bgRunner) shutdown(wait bool) {
 	}
 }
 
+// run calls `f` once in background.
 func (r *bgRunner) run(f func(context.Context)) {
 	if r.closed() {
 		return
@@ -562,6 +563,7 @@ func (r *bgRunner) run(f func(context.Context)) {
 	}()
 }
 
+// schedule calls `f` every `interval`.
 func (r *bgRunner) schedule(f func(context.Context, time.Time) bool, interval time.Duration) {
 	if r.closed() {
 		return
@@ -586,6 +588,7 @@ func (r *bgRunner) schedule(f func(context.Context, time.Time) bool, interval ti
 	}()
 }
 
+// scheduleWithTrigger likes schedule, but also call `f` when `<-trigger`, in which case the time arg of `f` is zero.
 func (r *bgRunner) scheduleWithTrigger(f func(context.Context, time.Time) bool, interval time.Duration, trigger <-chan struct{}) {
 	if r.closed() {
 		return