[Doc] Figure out and clarify docs for array

[karenzone]

Rob: The documentation at https://www.elastic.co/guide/en/logstash/current/configuration-file-structure.html#array talks about the array type being largely deprecated, and that isn't really borne out in practice - compare the number of occurrences of config options where :list => :true is used vs :validate => :array is used

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

Related issue: #11751
Reference: 7203

[karenzone]

cc:/ @111andre111 @robbavey