Doc: Clarify what happens with add_field mutation when field exists

[karenzone]

From Rob:
With the add_field mutation, if a field already exists, the new value will be appended to the end of the previous value as an array. The documentation for add_field doesn't mention that fact at all.

It seems like there are a couple of areas for improvements here - being explicit in our documentation around what happens with the add_field mutation when the field already exists - that it can transform an expected string or number into an array, and also clear up the story around arrays and the list property.

From Andre:
I took a look at the github repos and hope that are the places to be edited:
../../../../logstash/docs/include/input.asciidoc
{logstash-ref}/configuration-file-structure.html#string
type is string
#L139 https://github.com/elastic/logstash/edit/master/docs/include/input.asciidoc
Array Deprecated
#L89 https://github.com/elastic/logstash/edit/master/docs/static/configuration.asciidoc
String no mention that this can become an array as well
#253 https://github.com/elastic/logstash/edit/master/docs/static/configuration.asciidoc
add_field ../../../../logstash/docs/include/filter.asciidoc
#42 - #73 https://github.com/elastic/logstash/edit/master/docs/include/filter.asciidoc

Reference: 7203

[karenzone]

cc:/ @111andre111 @robbavey

[sophiaxu8]

@karenzone , any plan/update on this issue ?

[karenzone]

Thanks for the ping, @sophiaxu8. I'll take a fresh look at this one and see if we can get it prioritized.