ESQL: supplement docs on LIMIT (#125839)

This adds a few extra details around how ESQL processes input docs and
how it limits output results.

Closes #125819

docs/reference/query-languages/esql/limitations.md

@@ -10,20 +10,54 @@ mapped_pages:
 
 ## Result set size limit [esql-max-rows]
 
-By default, an {{esql}} query returns up to 1000 rows. You can increase the number of rows up to 10,000 using the [`LIMIT`](/reference/query-languages/esql/esql-commands.md#esql-limit) command. Queries do not return more than 10,000 rows, regardless of the `LIMIT` command’s value.
+By default, an {{esql}} query returns up to 1,000 rows. You can increase the number of rows up to 10,000 using the [`LIMIT`](/reference/query-languages/esql/esql-commands.md#esql-limit) command.
 
-This limit only applies to the number of rows that are retrieved by the query. Queries and aggregations run on the full data set.
+For instance,
+```esql
+FROM index | WHERE field = "value"
+```
+is equivalent to:
+```esql
+FROM index | WHERE field = "value" | LIMIT 1000
+```
+
+Queries do not return more than 10,000 rows, regardless of the `LIMIT` command’s value. This is a configurable upper limit.
 
 To overcome this limitation:
 
 * Reduce the result set size by modifying the query to only return relevant data. Use [`WHERE`](/reference/query-languages/esql/esql-commands.md#esql-where) to select a smaller subset of the data.
 * Shift any post-query processing to the query itself. You can use the {{esql}} [`STATS`](/reference/query-languages/esql/esql-commands.md#esql-stats-by) command to aggregate data in the query.
 
+The upper limit only applies to the number of rows that are output by the query, not to the number of documents it processes: the query runs on the full data set.
+
+Consider the following two queries:
+```esql
+FROM index | WHERE field0 == "value" | LIMIT 20000
+```
+and
+```esql
+FROM index | STATS AVG(field1) BY field2 | LIMIT 20000
+```
+
+In both cases, the filtering by `field0` in the first query or the grouping by `field2` in the second is applied over all the documents present in the `index`, irrespective of their number or indexes size. However, both queries will return at most 10,000 rows, even if there were more rows available to return.
+
 The default and maximum limits can be changed using these dynamic cluster settings:
 
 * `esql.query.result_truncation_default_size`
 * `esql.query.result_truncation_max_size`
 
+However, doing so involves trade-offs. A larger result-set involves a higher memory pressure and increased processing times; the internode traffic within and across clusters can also increase.
+
+These limitations are similar to those enforced by the [search API for pagination](/reference/elasticsearch/rest-apis/paginate-search-results.md#paginate-search-results).
+
+| Functionality                    | Search                  | {{esql}}                                  |
+|----------------------------------|-------------------------|-------------------------------------------|
+| Results returned by default      | 10                      | 1.000                                     |
+| Default upper limit              | 10,000                  | 10,000                                    |
+| Specify number of results        | `size`                  | `LIMIT`                                   |
+| Change default number of results | n/a                     | esql.query.result_truncation_default_size |
+| Change default upper limit       | index-max-result-window | esql.query.result_truncation_max_size     |
+
 
 ## Field types [esql-supported-types]