The StemmerTokenFilter had a number of issues:

* `english` returned the slow snowball English stemmer
* `porter2` returned the snowball Porter stemmer (v1)
* `portuguese` was used twice, preventing the second version from working

Changes:

* `english` now returns the fast PorterStemmer (for indices created from v1.3.0 onwards)
* `porter2` now returns the snowball English stemmer (for indices created from v1.3.0 onwards)
* `light_english` now returns the `kstem` stemmer (`kstem` still works)
* `portuguese_rslp` returns the PortugueseStemmer
* `dutch_kp` is a synonym for `kp`

Tests and docs updated

Fixes #6345
Fixes #6213
Fixes #6330