Added a note about same dtypes in expressions

Author: FrancescAlted

src/blosc2/lazyexpr.py

@@ -1294,6 +1294,9 @@ def fast_eval(  # noqa: C901
 
     if use_miniexpr:
         op_dtypes = {op.dtype for op in operands.values() if isinstance(op, blosc2.NDArray)}
+        # This is for avoiding type casting issues in miniexpr, like in:
+        # tests/ndarray/test_lazyexpr_fields.py::test_where_fusion6[dtype_fixture0-shape_fixture0-chunks_blocks_fixture0]
+        # TODO: remove this restriction when miniexpr supports type casting better
         if len(op_dtypes) > 1:
             use_miniexpr = False
         # Avoid padding issues except for 1D arrays (contiguous along the only axis).