docs: clarify type requirement for vectors for Postgres and Qdrant. (#789)

Author: badmonster0

docs/docs/ops/targets.md

@@ -32,6 +32,13 @@ Here's how CocoIndex data elements map to Postgres elements during export:
 For example, if you have a data collector that collects rows with fields `id`, `title`, and `embedding`, it will be exported to a Postgres table with corresponding columns.
 It should be a unique table, meaning that no other export target should export to the same table.
 
+:::warning vector type mapping to Postgres
+
+Since vectors in pgvector must have fixed dimension, we only map vectors of number types with fixed dimension (i.e. *Vector[cocoindex.Float32, N]*, *Vector[cocoindex.Float64, N]*, and *Vector[cocoindex.Int64, N]*) to `vector(N)` columns.
+For all other vector types, we map them to `jsonb` columns.
+
+:::
+
 #### Spec
 
 The spec takes the following fields:
@@ -58,6 +65,13 @@ Here's how CocoIndex data elements map to Qdrant elements during export:
 
 *Vector[Float32, N]*, *Vector[Float64, N]* and *Vector[Int64, N]* types fit into Qdrant vector.
 
+:::warning vector type mapping to Qdrant
+
+Since vectors in Qdrant must have fixed dimension, we only map vectors of number types with fixed dimension (i.e. *Vector[cocoindex.Float32, N]*, *Vector[cocoindex.Float64, N]*, and *Vector[cocoindex.Int64, N]*) to Qdrant vectors.
+For all other vector types, we map to Qdrant payload as JSON arrays.
+
+:::
+
 #### Spec
 
 The spec takes the following fields: