doc: mention returned error types and values on the net package in go1.5.txt

Change-Id: I70dfc2bad13c513c376c7c41058774b40af73dce
Reviewed-on: https://go-review.googlesource.com/9775Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/go1.5.txt

@@ -52,6 +52,7 @@ math/big: add Jacobi and Int.ModSqrt (https://golang.org/cl/1886)
 mime: add ExtensionByType (https://golang.org/cl/7444)
 mime/quotedprintable: new package (https://golang.org/cl/5940 + others)
 net: add Source field to OpError (https://go-review.googlesource.com/9231)
+net: fix inconsistent errors (https://golang.org/cl/9236)
 net/http: support for setting trailers from a server Handler (https://golang.org/cl/2157)
 net/http: ignore the Unix epoch time in ServeContent (https://golang.org/cl/7915)
 net/http/cgi: fix REMOTE_ADDR, REMOTE_HOST, add REMOTE_PORT (https://golang.org/cl/4933)