[CALCITE-6250] Limitations of MongoDB adapter are not documented
Add hints with restrictions for documents/filters to howto site
diff --git a/site/_docs/howto.md b/site/_docs/howto.md
index a6abb4f..15a7c6c 100644
--- a/site/_docs/howto.md
+++ b/site/_docs/howto.md
@@ -329,7 +329,10 @@
First, download and install Calcite,
and <a href="https://www.mongodb.org/downloads">install MongoDB</a>.
-Note: you can use MongoDB from the integration test virtual machine above.
+Note:
+* You can use MongoDB from the integration test virtual machine above.
+* The MongoDB adapter is not suited for querying nested documents.
+* The MongoDB adapter currently only supports a very limited set of filter push downs.
Import MongoDB's zipcode data set into MongoDB:
