encoding: be clear what is returned in error cases
encoding: be clear what is returned in error cases

Several methods return a zero value, err pair in the error case. The old
documentation  described this as "0, err" which is a bit misleading as 0
is neither a string nor a byte slice. Improve the documentation by being
specific what is returned.

Change-Id: If75af8078149e7109cb6e70cf5f4d1eebca4fdec
Reviewed-on: https://go-review.googlesource.com/27090
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

golang/text