Add a SQL example for `not` [ci skip]

To share a certain logic across other examples, let's add a sample SQL
code generated by the given Ruby code

guides/source/active_record_querying.md

@@ -514,7 +514,13 @@ SELECT * FROM clients WHERE (clients.orders_count IN (1,3,5))
 Post.where.not(author: author)
 ```
 
-In other words, this query can be generated by calling `where` with no argument, then immediately chain with `not` passing `where` conditions.
+In other words, this query can be generated by calling `where` with no argument,
+then immediately chain with `not` passing `where` conditions. This will generate
+SQL code like this:
+
+```sql
+SELECT * FROM posts WHERE (author_id != 1)
+```
 
 Ordering
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