Added a docstring for the new signing method

Author: susperius

Diff for: sigstore/sign.py

@@ -233,6 +233,17 @@ def sign_dsse_envelope(
             self,
             envelope: dsse.Envelope,
     ) -> Bundle:
+        """
+        Signs the provided envelope's payload, and returns a
+        `Bundle containing the signed envelope and the verification
+        material.
+
+        Args:
+            envelope (dsse.Envelope): The envelope to be signed.
+
+        Returns:
+            Bundle: The bundle containing the signed DSSE envelope.
+        """
         cert = self._signing_cert()
 
         b64_cert = base64.b64encode(