Update indices-query.asciidoc

The current description of the query seemed confusing to some people (e.g. https://twitter.com/wielinde/status/654582620630687744), this is just a proposal to simplify it.

docs/reference/query-dsl/indices-query.asciidoc

@@ -1,10 +1,11 @@
 [[query-dsl-indices-query]]
 === Indices Query
 
-The `indices` query can be used when executed across multiple indices,
-allowing to have a query that executes only when executed on an index
-that matches a specific list of indices, and another query that executes
-when it is executed on an index that does not match the listed indices.
+The `indices` query is useful in cases where a search is executed across
+multiple indices. It allows to specify a list of index names and an inner
+query that is only executed for indices matching names on that list.
+For other indices that are searched, but that don't match entries
+on the list, the alternative `no_match_query` is exectuted.
 
 [source,js]
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