net/http/httptrace: clarify ClientTrace docs

The old wording over-promised.

Fixes #16957

Change-Id: Iaac04de0d24eb17a0db66beeeab9de70d0f6d391
Reviewed-on: https://go-review.googlesource.com/31735Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>

src/net/http/httptrace/trace.go

@@ -68,9 +68,8 @@ func WithClientTrace(ctx context.Context, trace *ClientTrace) context.Context {
 
 // ClientTrace is a set of hooks to run at various stages of an outgoing
 // HTTP request. Any particular hook may be nil. Functions may be
-// called concurrently from different goroutines, starting after the
-// call to Transport.RoundTrip and ending either when RoundTrip
-// returns an error, or when the Response.Body is closed.
+// called concurrently from different goroutines and some may be called
+// after the request has completed or failed.
 type ClientTrace struct {
 	// GetConn is called before a connection is created or
 	// retrieved from an idle pool. The hostPort is the