webdav: fill in the package's doc comment.

Change-Id: I078768667f46d413a65a347185a8fb296b9c5b1e
Reviewed-on: https://go-review.googlesource.com/21633
Reviewed-by: Andrew Gerrand <adg@golang.org>

webdav/webdav.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package webdav etc etc TODO.
+// Package webdav provides a WebDAV server implementation.
 package webdav // import "golang.org/x/net/webdav"
 
 import (