net/rpc: move frozen notice to the start of the package doc

For #71559

Change-Id: I68b9518a26cab75789d596839267abab7997bc2c
Reviewed-on: https://go-review.googlesource.com/c/go/+/646575
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>

src/net/rpc/server.go

@@ -4,7 +4,11 @@
 
 /*
 Package rpc provides access to the exported methods of an object across a
-network or other I/O connection.  A server registers an object, making it visible
+network or other I/O connection.
+
+The net/rpc package is frozen and is not accepting new features.
+
+A server registers an object, making it visible
 as a service with the name of the type of the object.  After registration, exported
 methods of the object will be accessible remotely.  A server may register multiple
 objects (services) of different types but it is an error to register multiple
@@ -121,8 +125,6 @@ or
 
 A server implementation will often provide a simple, type-safe wrapper for the
 client.
-
-The net/rpc package is frozen and is not accepting new features.
 */
 package rpc