docs: show how to run Functions and BlobReader locally (#4803)

Running a Function body in a local test or another engine currently
requires discovering undocumented APIs. Document direct calls to
decorated Functions and classes, including initialization fields,
instance reuse, and cleanup, and explain how they differ from registered
`FunctionVersion` applications.

Add examples of `BlobReader(read_range, size)` backed by an in-memory
byte-range callback, including cursor-independent reads. The local
examples and rendered API documentation were validated on EC2 at
`df6076e1ae605f425a380d282c4f25b845a3165f`; integration with current
main leaves these documented call paths unchanged.

Fixes ENT-3561.

docs/src/python/python.md

@@ -377,11 +377,64 @@ code that used the former LanceDB Enterprise-only argument order.
 
 ## Functions and Jobs
 
+A decorated `udf` definition is also a local callable. Call a decorated function
+directly with row inputs, or call a decorated class with its initialization fields
+and then call the returned instance; call `close()` if the class defines it. This
+path uses the current Python environment and does not create a registered
+`FunctionVersion`, build the declared remote runtime, resolve Secrets, or execute
+the remote Arrow adapter. A registered `FunctionVersion` call instead binds
+`col(...)` inputs and returns a remote `FunctionApplication`.
+
+```python
+from lancedb import udf
+
+
+@udf
+def normalize(text: str) -> str:
+    return text.strip().lower()
+
+
+@udf
+class Scale:
+    def __init__(self, factor: float):
+        self.factor = factor
+
+    def __call__(self, value: float) -> float:
+        return value * self.factor
+
+    def close(self) -> None:
+        self.factor = 0.0
+
+
+assert normalize(" Hello ") == "hello"
+scaler = Scale(factor=2.0)
+assert scaler(3.0) == 6.0
+scaler.close()
+```
+
 Annotate a blob input with `lancedb.BlobReader` for lazy, seekable access, or
 with `bytes` to materialize its contents before the Function runs. Reader
 inputs infer the Blob V2 type without an explicit input schema. Readers are
 scoped to each invocation and support `read`, `readinto`, `seek`, `tell`,
-`size`, and cursor-independent `read_range`.
+`size`, and cursor-independent `read_range`. For a local test or another
+engine, construct `BlobReader(read_range, size)` with a callback over an
+in-memory bytes fixture or the engine's byte-range reader; `read_range` does
+not advance the cursor.
+
+```python
+from lancedb import BlobReader
+
+payload = b"0123456789"
+
+
+def read_range(offset: int, length: int) -> bytes:
+    return payload[offset : offset + length]
+
+
+with BlobReader(read_range, len(payload)) as reader:
+    assert reader.read_range(6, 4) == b"6789"
+    assert reader.tell() == 0
+```
 
 ::: lancedb.BlobReader