docs(python): note that packaged Functions import lance_blob.BlobReader (#4730)

Function authors annotate inputs with lancedb.BlobReader, but the
packaged source binds that annotation to the worker's
lance_blob.BlobReader, so grepping the artifact for the client class
finds nothing. Say so on the class.

python/python/lancedb/_blob_reader.py

@@ -21,6 +21,9 @@ class BlobReader(io.RawIOBase):
     ``read_range`` does not change the sequential cursor. Readers are scoped
     to the invocation and must not be retained for a later call.
 
+    A packaged Function's source imports the worker's ``lance_blob.BlobReader``
+    in place of this class.
+
     For local calls, construct a reader from a range-reading callable and the
     blob's size. The callable accepts an offset and length and returns bytes.
     """