resolver/dns: Add docstring to SetMinResolutionInterval (#7217)

Author: arvindbr8

resolver/dns/dns_resolver.go

@@ -18,9 +18,6 @@
 
 // Package dns implements a dns resolver to be installed as the default resolver
 // in grpc.
-//
-// Deprecated: this package is imported by grpc and should not need to be
-// imported directly by users.
 package dns
 
 import (
@@ -53,8 +50,11 @@ func NewBuilder() resolver.Builder {
 	return dns.NewBuilder()
 }
 
-// SetMinResolutionInterval sets the default minimum interval at which DNS re-resolutions are
-// allowed. This helps to prevent excessive re-resolution.
+// SetMinResolutionInterval sets the default minimum interval at which DNS
+// re-resolutions are allowed. This helps to prevent excessive re-resolution.
+//
+// It must be called only at application startup, before any gRPC calls are
+// made. Modifying this value after initialization is not thread-safe.
 func SetMinResolutionInterval(d time.Duration) {
 	dns.MinResolutionInterval = d
 }