doc/go1.1.html, mime/multipart: document quoted-printable decoding

Fixes #5293

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8840043

doc/go1.1.html

@@ -862,6 +862,9 @@ has a new method for its
 <a href="/pkg/mime/multipart/#Writer"><code>Writer</code></a>,
 <a href="/pkg/mime/multipart/#Writer.SetBoundary"><code>SetBoundary</code></a>,
 to define the boundary separator used to package the output.
+The <a href="/pkg/mime/multipart/#Reader"><code>Reader</code></a> also now
+transparently decodes any <code>quoted-printable</code> parts and removes
+the <code>Content-Transfer-Encoding</code> header when doing so.
 </li>
 
 <li>

src/pkg/mime/multipart/multipart.go

@@ -28,7 +28,12 @@ var emptyParams = make(map[string]string)
 type Part struct {
 	// The headers of the body, if any, with the keys canonicalized
 	// in the same fashion that the Go http.Request headers are.
-	// i.e. "foo-bar" changes case to "Foo-Bar"
+	// For example, "foo-bar" changes case to "Foo-Bar"
+	//
+	// As a special case, if the "Content-Transfer-Encoding" header
+	// has a value of "quoted-printable", that header is instead
+	// hidden from this map and the body is transparently decoded
+	// during Read calls.
 	Header textproto.MIMEHeader
 
 	buffer    *bytes.Buffer