io: clarify documentation for Seeker

io: clarify documentation for Seeker

The later part of the docstring simply talks about "offset" but does
not disambiguate what it is relative to. For both the return value
and valid offsets to seek to, it only makes sense in the context of
"offset relative to origin of file".

Fixes #11877

Change-Id: Ic238a407cf8e8fdd64991d98a6584cdc8a51cd6b
Reviewed-on: https://go-review.googlesource.com/15257
Reviewed-by: Andrew Gerrand <adg@golang.org>