[DOCS][ESQL] Include bucket in agg functions list (elastic#112513)

(cherry picked from commit 04678e9)

# Conflicts:
#	docs/reference/esql/processing-commands/stats.asciidoc

Author: leemthompo

docs/reference/esql/processing-commands/stats.asciidoc

@@ -2,6 +2,9 @@
 [[esql-stats-by]]
 === `STATS ... BY`
 
+The `STATS ... BY` processing command groups rows according to a common value
+and calculates one or more aggregated values over the grouped rows.
+
 **Syntax**
 
 [source,esql]
@@ -38,6 +41,10 @@ The following <<esql-agg-functions,aggregation functions>> are supported:
 
 include::../functions/aggregation-functions.asciidoc[tag=agg_list]
 
+The following <<esql-group-functions,grouping functions>> are supported:
+
+include::../functions/grouping-functions.asciidoc[tag=group_list]
+
 NOTE: `STATS` without any groups is much much faster than adding a group.
 
 NOTE: Grouping on a single expression is currently much more optimized than grouping