docs(limiter/conn): add doc comments, fix nil guards and missing Close

Add package and exported symbol doc comments across conn, generator,
limiter, and wrapper packages. Fix nil receiver guard in
connLimitSingleGenerator.Limiter, add missing httpLoader.Close call,
return context.Background instead of nil in serverConn.Context, return
explicit error on connection limit exceeded, and simplify Allow logic.

limiter/conn/limiter.go

@@ -12,6 +12,7 @@ type llimiter struct {
 	current int64
 }
 
+// NewLimiter creates a Limiter that allows up to n concurrent connections.
 func NewLimiter(n int) limiter.Limiter {
 	return &llimiter{limit: n}
 }
@@ -21,10 +22,12 @@ func (l *llimiter) Limit() int {
 }
 
 func (l *llimiter) Allow(n int) bool {
+	if n < 0 {
+		atomic.AddInt64(&l.current, int64(n))
+		return true
+	}
 	if atomic.AddInt64(&l.current, int64(n)) > int64(l.limit) {
-		if n > 0 {
-			atomic.AddInt64(&l.current, -int64(n))
-		}
+		atomic.AddInt64(&l.current, -int64(n))
 		return false
 	}
 	return true