Docs: Update execution hint docs for Significant terms agg

copied over the relevant pieces from the terms agg

Closes elastic#8532

docs/reference/search/aggregations/bucket/significantterms-aggregation.asciidoc

@@ -474,12 +474,29 @@ http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html#UNIX_LINES
 
 ===== Execution hint
 
-There are two mechanisms by which terms aggregations can be executed: either by using field values directly in order to aggregate
-data per-bucket (`map`), or by using ordinals of the field values instead of the values themselves (`ordinals`). Although the
-latter execution mode can be expected to be slightly faster, it is only available for use when the underlying data source exposes
-those terms ordinals. Moreover, it may actually be slower if most field values are unique. Elasticsearch tries to have sensible
-defaults when it comes to the execution mode that should be used, but in case you know that an execution mode may perform better
-than the other one, you have the ability to provide Elasticsearch with a hint:
+added[1.2.0] Added the `global_ordinals`, `global_ordinals_hash` and `global_ordinals_low_cardinality` execution modes
+
+deprecated[1.3.0] Removed the `ordinals` execution mode
+
+There are different mechanisms by which terms aggregations can be executed:
+
+ - by using field values directly in order to aggregate data per-bucket (`map`)
+ - by using ordinals of the field and preemptively allocating one bucket per ordinal value (`global_ordinals`)
+ - by using ordinals of the field and dynamically allocating one bucket per ordinal value (`global_ordinals_hash`)
+
+Elasticsearch tries to have sensible defaults so this is something that generally doesn't need to be configured.
+
+`map` should only be considered when very few documents match a query. Otherwise the ordinals-based execution modes
+are significantly faster. By default, `map` is only used when running an aggregation on scripts, since they don't have
+ordinals.
+
+`global_ordinals` is the second fastest option, but the fact that it preemptively allocates buckets can be memory-intensive,
+especially if you have one or more sub aggregations. It is used by default on top-level terms aggregations.
+
+`global_ordinals_hash` on the contrary to `global_ordinals` and `global_ordinals_low_cardinality` allocates buckets dynamically
+so memory usage is linear to the number of values of the documents that are part of the aggregation scope. It is used by default
+in inner aggregations.
+
 
 [source,js]
 --------------------------------------------------
@@ -495,6 +512,7 @@ than the other one, you have the ability to provide Elasticsearch with a hint:
 }
 --------------------------------------------------
 
-<1> the possible values are `map` and `ordinals`
+<1> the possible values are `map`, `global_ordinals` and `global_ordinals_hash`
 
 Please note that Elasticsearch will ignore this execution hint if it is not applicable.
+