Warn when `quantiles` passed to `survey_median()`

[gergness]

Seems like an easy mistake to make, and currently it just silently gives you the median.

library(srvyr)
data(api, package = "survey")

dstrata <- apistrat %>%
   as_survey(strata = stype, weights = pw)

dstrata %>%
   summarize(
      x = survey_median(api99, quantiles = 0.2),
      median = survey_median(api99),
      twentith = survey_quantile(api99, quantiles = 0.2)
   )
#> # A tibble: 1 × 6
#>       x  x_se median median_se twentith_q20 twentith_q20_se
#>   <dbl> <dbl>  <dbl>     <dbl>        <dbl>           <dbl>
#> 1   631  17.5    631      17.5          504            10.6