fix fswap docstring

python/ffsim/gates/basic_gates.py

@@ -581,9 +581,18 @@ def apply_fswap_gate(
     *,
     copy: bool = True,
 ) -> np.adarray:
-    r"""Apply a fSWAP gate 
+    r"""Apply a fSWAP gate that swaps target orbitals.
 
-        .. math::
+    An fSwap gate is defined as
+
+    .. math::
+        f_\text{swap}(p, q) = 
+        1 + a^\dagger_p a_q + a^\dagger_q a_p - a_p^\dagger a_p - a_q^\dagger a_q
+
+    Under the Jordan-Wigner transform, this gate has the following matrix when applied 
+    to neighboring qubits: 
+
+    .. math::
 
         \begin{pmatrix}
             1 & 0 & 0 & 0 \\