Documented [] syntax for buckets_path

Closes #15707

docs/reference/aggregations/pipeline.asciidoc

@@ -104,7 +104,7 @@ a metric embedded inside a sibling aggregation:
 `sales_per_month` date histogram.
 
 [float]
-==== Special Paths
+=== Special Paths
 
 Instead of pathing to a metric, `buckets_path` can use a special `"_count"` path.  This instructs
 the pipeline aggregation to use the document count as it's input.  For example, a moving average can be calculated on the document
@@ -128,6 +128,20 @@ count of each bucket, instead of a specific metric:
 --------------------------------------------------
 <1> By using `_count` instead of a metric name, we can calculate the moving average of document counts in the histogram
 
+[[dots-in-agg-names]]
+[float]
+=== Dealing with dots in agg names
+
+An alternate syntax is supported to cope with aggregations or metrics which
+have dots in the name, such as the ++99.9++th
+<<search-aggregations-metrics-percentile-aggregation,percentile>>. This metric
+may be referred to as:
+
+[source,js]
+---------------
+"buckets_path": "my_percentile[99.9]"
+---------------
+
 [[gap-policy]]
 [float]
 === Dealing with gaps in the data