The indices query allows the user to run different queries on different indices, eg to create a union of docs from different indices, where the conditions to be applied to each index are different.

A simpler way to write this query would be to make _index usable in the term, terms, match, query_string, and simple_query_stringqueries (see #3316).

However, this change also depends on the changes in #12016 to allow unmapped fields to be handled gracefully across all queries.