Add notes to project_operator docstring

qiskit_addon_sqd/qubit.py

@@ -104,6 +104,18 @@ def project_operator_to_subspace(
     The subspace is defined by a collection of computational basis states, which are specified by the bitstrings
     (rows) in ``bitstring_matrix``.
 
+    .. note::
+       The bitstrings in the ``bitstring_matrix`` must be unique and sorted in ascending order
+       according to their unsigned integer representation. Otherwise the projection will return wrong
+       results. This function does not explicitly check for uniqueness and order because
+       this can be rather time consuming. See :func:`qiskit_addon_sqd.qubit.sort_and_remove_duplicates`
+       for a simple way to ensure your bitstring matrix is well-formatted.
+
+    .. note::
+       This function relies on ``jax`` to efficiently perform some calculations. ``jax``
+       converts the bit arrays to ``int64_t``, which means the bit arrays in
+       ``bitstring_matrix`` may not have length greater than ``63``.
+
     Args:
         bitstring_matrix: A 2D array of ``bool`` representations of bit
             values such that each row represents a single bitstring. This set of