docs: describe more explicitly what happens when indexing queries that fetch terms

docs/reference/mapping/types/percolator.asciidoc

@@ -82,4 +82,7 @@ time (using `now`).
 There are a number of queries that fetch data via a get call during query parsing. For example the `terms` query when
 using terms lookup, `template` query when using indexed scripts and `geo_shape` when using pre-indexed shapes. When these
 queries are indexed by the `percolator` field type then the get call is executed once. So each time the `percolator`
-query evaluates these queries, the fetches terms, shapes etc. as the were upon index time will be used.
+query evaluates these queries, the fetches terms, shapes etc. as the were upon index time will be used. Important to note
+is that fetching of terms that these queries do, happens both each time the percolator query gets indexed on both primary
+and replica shards, so the terms that are actually indexed can be different between shard copies, if the source index
+changed while indexing.