encoding/xml: document behavior for undefined name space prefixes
encoding/xml: document behavior for undefined name space prefixes

Fixes issue 5626.

R=golang-dev, dominik.honnef
CC=golang-dev
https://codereview.appspot.com/13702043

pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang