: improved discoverability in godoc

It seems that people tend to search for terms like
i18n and l10n to find packages with related
funcionality. None of the text packages pop up
for such a search.
This change adds these terms to the root package
at least.

Change-Id: I3a84a51741ebf353ea21cc53c9c2fbf6140b32d6
Reviewed-on: https://go-review.googlesource.com/25052
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>