[docs] _csvparsefailure tag behavior is not documented

[andrewkroh]

Description:

The failure behavior of the filter is not documented. The documentation should mention that _csvparsefailure is added to tags upon failure.

logstash-filter-csv/lib/logstash/filters/csv.rb

Line 171 in 30b2c23

event.tag("_csvparsefailure")