Add documents to clarify indexable types and vector indexing metrics. (#52)

Author: badmonster0

docs/docs/core/data_types.mdx

@@ -43,4 +43,30 @@ A struct has a bunch of fields, each with a name and a type.
 
 A table has a collection of rows, each of which is a struct with specified schema.
 
-The first field of a table is always the primary key.
+The first field of a table is always the primary key.
+
+## Indexable Types
+
+### Key Types
+
+Currently, the following types are supported as types for key fields:
+
+- `bytes`
+- `str`
+- `bool`
+- `int64`
+- `range`
+- Struct with all fields being key types
+
+### Vector Type
+
+Users can create vector index on fields with `vector` types.
+A vector index also needs to be configured with a similarity metric, and the index is only effective when this metric is used during retrieval.
+
+Following metrics are supported:
+
+| Metric Name | Description | Similarity Order |
+|-------------|-------------|------------------|
+| `CosineSimilarity` | [Cosine similarity](https://en.wikipedia.org/wiki/Cosine_similarity) | Larger is more similar |
+| `L2Distance` | [L2 distance (a.k.a. Euclidean distance)](https://en.wikipedia.org/wiki/Euclidean_distance) | Smaller is more similar |
+| `InnerProduct` | [Inner product](https://en.wikipedia.org/wiki/Inner_product_space) | Larger is more similar |

docs/docs/core/flow_def.mdx

@@ -198,8 +198,8 @@ Export must happen at the top level of a flow, i.e. not within any child scopes
 
 *   `name`: the name to identify the export target.
 *   `target_spec`: the storage spec as the export target.
-*   `primary_key_fields` (optional): the fields to be used as primary key.
-*   `vector_index` (optional): the fields to create vector index.
+*   `primary_key_fields` (optional): the fields to be used as primary key. Types of the fields must be supported as key fields. See [Key Types](data_types#key-types) for more details.
+*   `vector_index` (optional): the fields to create vector index. Each item is a tuple of a field name and a similarity metric. See [Vector Type](data_types#vector-type) for more details about supported similarity metrics.
 
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