This commit fixes two issues with the slow log docs:
- clarifies that these settings are per index
- updates index slow log configuration for Log4j 2
Relates #20976
Instead of logging the entire `_source` in the indexing slowlog we log by
default just the first 1000 characters - this is controlled by the
`index.indexing.slowlog.source` settings and can be set to `true` to log the
whole `_source`, `false` to log none of it, and a number to log at most that
many characters.
Closes#4485
Jason Tedor
Sakthipriyan Vairamani
Nik Everett
Clinton Gormley
Kouhei Sutou
Lee Hinman