PPL manual update

dashboards-observability/public/components/common/helpers/ppl_docs/functions/relevance.ts

@@ -61,4 +61,22 @@ parameters:
     |------------|
     | Bond       |
     +------------+
+
+### Limitations
+
+The relevance functions are available to execute only in OpenSearch DSL
+but not in memory as of now, so the relevance search might fail for
+queries that are too complex to translate into DSL if the relevance
+function is following after a complex PPL query. To make your queries
+always work-able, it is recommended to place the relevance commands as
+close to the search command as possible, to ensure the relevance
+functions are eligible to push down. For example, a complex query like
+\`search source = people | rename firstname as name | dedup account_number | fields name, account_number, balance, employer | where match(employer, 'Open Search') | stats count() by city\`
+could fail because it is difficult to translate to DSL, but it would be
+better if we rewrite it to an equivalent query as
+\`search source = people | where match(employer, 'Open Search') | rename firstname as name | dedup account_number | fields name, account_number, balance, employer | stats count() by city\`
+by moving the where command with relevance function to the second
+command right after the search command, and the relevance would be
+optimized and executed smoothly in OpenSearch DSL. See [Optimization](https://github.com/opensearch-project/sql/blob/22924b13d9cb46759c8d213a7ce903effe06ab47/docs/user/optimization/optimization.rst)
+to get more details about the query engine optimization.
 `;