Update supabase_js_v1_legacy.yml

Please see https://github.com/supabase/supabase-js/issues/550

spec/supabase_js_v1_legacy.yml

@@ -961,6 +961,7 @@ pages:
     notes: |
       - Primary keys should be included in the data payload in order for an update to work correctly.
       - Primary keys  must be natural, not surrogate. There are however, [workarounds](https://github.com/PostgREST/postgrest/issues/1118) for surrogate primary keys.
+      - It is currently not possible to insert new data and update existing data at the same time. However, a [workaround](https://github.com/supabase/postgrest-js/issues/173#issuecomment-825124550) can be employed using Postgres functions/triggers.
     examples:
       - name: Upsert your data
         isSpotlight: true