Lower threshold for strata pooling

[juliasilge]

This PR lowers the threshold for strata pooling from 15% of the total to 10% of the total. It also adds info that this pooling is happening to the .Rd files.

This new threshold seems to solve the problems users have reported. For example:

library(tidyverse)
library(rsample)

X <- matrix(rnorm(140 * 100), ncol = 100, nrow = 140)
y <- rep(letters[1:7], each = 20)

df <- tibble(X) %>%
  mutate(label = y)

vfold_cv(df, v = 3, strata = label)
#> #  3-fold cross-validation using stratification 
#> # A tibble: 3 x 2
#>   splits          id   
#>   <named list>    <chr>
#> 1 <split [91/49]> Fold1
#> 2 <split [91/49]> Fold2
#> 3 <split [98/42]> Fold3

^{Created on 2020-05-07 by the reprex package (v0.3.0)}

Notice there is no longer the warning about "too little data to stratify".

This PR closes #86, #91, and #110.

[juliasilge]

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