doc/effective_go: clarify advice on returning interfaces

New Gophers sometimes misconstrue the advice in the "Generality" section
as "export interfaces instead of implementations" and add needless
interfaces to their code as a result.  Down the road, they end up
needing to add methods and either break existing callers or have to
resort to unpleasant hacks (e.g. using "magic method" type-switches).

Weaken the first paragraph of this section to only advise leaving types
unexported when they will never need additional methods.

Change-Id: I32a1ae44012b5896faf167c02e192398a4dfc0b8
Reviewed-on: https://go-review.googlesource.com/24892


Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>