Small continuation to `sa` description

[pablobernabeu]

Just added and \code{"sa"} in:

The \code{method} argument can be used to specify one of the following tests.
Note that \code{"z"} is an asymptotic approximation for models not fitted
with \code{\link[lme4]{glmer}}, and \code{"kr"} and \code{"sa"} will only work
with models fitted with \code{\link[lme4]{lmer}}.

[pitakakariki]

Thanks.