net/http: clarify Request.ContentLength behavior on the client.

While you could argue the previous wording technically said that -1 is
an acceptable way to indicate "unknown" on the client, it could be read
as ambiguous. Now it's clear that both 0 and -1 mean unknown.

Change-Id: I3bc5a3fd5afd1999e487296ec121eb548415e6b0
Reviewed-on: https://go-review.googlesource.com/29130Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/request.go

@@ -153,7 +153,8 @@ type Request struct {
 	// The value -1 indicates that the length is unknown.
 	// Values >= 0 indicate that the given number of bytes may
 	// be read from Body.
-	// For client requests, a value of 0 means unknown if Body is not nil.
+	// For client requests, a value of 0 with a non-nil Body is
+	// also treated as unknown.
 	ContentLength int64
 
 	// TransferEncoding lists the transfer encodings from outermost to