[Doc] Update databricks_vector_search_index docs to match latest SDK (#4008)

## Changes
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Should be merged after #4007

## Tests
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- [ ] `make test` run locally
- [x] relevant change in `docs/` folder
- [ ] covered with integration tests in `internal/acceptance`
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Author: alexott

docs/resources/vector_search_index.md

@@ -38,6 +38,7 @@ The following arguments are supported (change of any parameter leads to recreati
   * `DIRECT_ACCESS`: An index that supports the direct read and write of vectors and metadata through our REST and SDK APIs. With this model, the user manages index updates.
 * `delta_sync_index_spec` - (object) Specification for Delta Sync Index. Required if `index_type` is `DELTA_SYNC`.
   * `source_table` (required) The name of the source table.
+  * `columns_to_sync` - (optional) list of columns to sync. If not specified, all columns are syncronized.
   * `embedding_source_columns` - (required if `embedding_vector_columns` isn't provided) array of objects representing columns that contain the embedding source.  Each entry consists of:
 	* `name` - The name of the column
 	* `embedding_model_endpoint_name` - The name of the embedding model endpoint