ESQL: Explain confusing query

This links to an explanation about a very confusing query that you can
write in ESQL. It've decided to link to github here to because this
query is so deeply at the edge that you need to know that you are
dealing with hackers now.

Closes #134792

Author: nik9000

docs/reference/query-languages/esql/_snippets/commands/layout/stats-by.md

@@ -75,7 +75,7 @@ and then grouping - that is not going to be faster.
 ::::
 
 
-**Examples**
+### Examples
 
 Calculating a statistic and grouping by the values of another column:
 
@@ -105,6 +105,8 @@ optional as well:
 :::
 
 $$$esql-stats-mv-group$$$
+#### Multivalued
+
 If the grouping key is multivalued then the input row is in all groups:
 
 :::{include} ../examples/stats.csv-spec/mv-group.md
@@ -147,6 +149,12 @@ letter of their last name:
 :::{include} ../examples/stats.csv-spec/docsStatsByExpression.md
 :::
 
+Note: This can get confusing if you do something like `| STATS a + SUM(a) BY a`
+      if `a` is multivalued. If you do that and are having trouble see
+      [github](https://github.com/elastic/elasticsearch/issues/134792#issuecomment-3361168090).
+
+#### Naming
+
 Specifying the output column name is optional. If not specified, the new column
 name is equal to the expression. The following query returns a column named
 `AVG(salary)`: