encoding/xml, encoding/json: docs and examples using custom marshalers

Both the encoding/xml and encoding/json packages support custom
marshalers for JSON and XML, as well as the basic encoding.TextMarshaler
and encoding.TextUnmarshaler interfaces, but the docs and examples for
these are missing.

There are docs for how to use encoding.TextMarshaler and
encoding.TextUnmarshaler in encoding/json, but not encoding/xml. There
are no examples for how to use them with either json or xml. This commit
includes docs for encoding/xml and examples for both encoding/json and
encoding/xml.

There is an example using custom marshalers MarshalJSON and
UnmarshalJSON in encoding/json, but not MarshalXML and UnmarshalXML in
encoding/json. These docs are more so necessary for encoding/xml because
the complexities of XML documents is significantly greater than JSON
documents which more often leads to the need for custom marshaling. The
encoding/json package includes an example of how to write a custom
marshaler, and this commit includes the same example for the xml
package.

All examples are mirrored off the existing custom marshaler example in
encoding/json.

Fixes #6859

Change-Id: Ic93abc27c0b4d5e48dea6ede4e20b1bedca4ab39
Reviewed-on: https://go-review.googlesource.com/c/76350
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/xml/marshal.go

@@ -61,6 +61,10 @@ const (
 //       string of length zero.
 //     - an anonymous struct field is handled as if the fields of its
 //       value were part of the outer struct.
+//     - a field implementing Marshaler is written by calling its MarshalXML
+//       method.
+//     - a field implementing encoding.TextMarshaler is written by encoding the
+//       result of its MarshalText method as text.
 //
 // If a field uses a tag "a>b>c", then the element c will be nested inside
 // parent elements a and b. Fields that appear next to each other that name