Add known limit on locality optimized search

Fixes #12175

_includes/v21.2/sql/locality-optimized-search-limited-records.md

@@ -0,0 +1,3 @@
+{% if page.name == "cost-based-optimizer.md" %} Locality-optimized search {% else %} [Locality-optimized search](cost-based-optimizer.html#locality-optimized-search-in-multi-region-clusters) {% endif %} only works for queries selecting a limited number of records (up to 100,000 unique keys). Also, It does not yet work with [`LIMIT`](limit-offset.html) clauses.
+
+[Tracking GitHub Issue](https://github.com/cockroachdb/cockroach/issues/64862)

v21.2/cost-based-optimizer.md

@@ -110,6 +110,10 @@ This can lead to increased performance in multi-region deployments, since it mea
 The asynchronous parallel lookup behavior described above does not occur if you [disable vectorized execution](vectorized-execution.html#configuring-vectorized-execution).
 {{site.data.alerts.end}}
 
+Note the following limitations:
+
+{% include {{ page.version.version }}/sql/locality-optimized-search-limited-records.md %}
+
 ## Query plan cache
 
 CockroachDB uses a cache for the query plans generated by the optimizer. This can lead to faster query execution since the database can reuse a query plan that was previously calculated, rather than computing a new plan each time a query is executed.

v21.2/known-limitations.md

@@ -173,6 +173,10 @@ UNION ALL SELECT * FROM t1 LEFT JOIN t2 ON st_contains(t1.geom, t2.geom) AND t2.
 
 {% include {{page.version.version}}/sql/jsonb-comparison.md %}
 
+### Locality-optimized search only works for queries selecting a limited number of records
+
+{% include {{ page.version.version }}/sql/locality-optimized-search-limited-records.md %}
+
 ## Unresolved limitations
 
 ### Optimizer stale statistics deletion when columns are dropped