encoding/xml, encoding/json: package docs don't mention encoding.TextMarshaler and encoding.TextUnmarshaler

[songgao]

version: Go 1.2

What is the expected output?
`encoding/xml` and `encoding/json` should document the behavior that it will check
encoding.TextMarshaler and encoding.TextUnmarshaler.

[dsymonds]

Comment 1:

Labels changed: added priority-soon, documentation, size-s, removed priority-triage.

Owner changed to @rsc.

Status changed to Accepted.

[rsc]

Comment 2:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 3:

Labels changed: added repo-main.