doc update

src/openpgp.js

@@ -182,6 +182,7 @@ export function decryptKey({ privateKey, passphrase }) {
  * @param  {Boolean} armor                    (optional) if the return values should be ascii armored or the message/signature objects
  * @param  {Boolean} detached                 (optional) if the signature should be detached (if true, signature will be added to returned object)
  * @param  {Signature} signature              (optional) a detached signature to add to the encrypted message
+ * @param  {Boolean} returnSessionKey         (optional) if the unencrypted session key should be added to returned object
  * @return {Promise<Object>}                  encrypted (and optionally signed message) in the form:
  *                                              {data: ASCII armored message if 'armor' is true,
  *                                                message: full Message object if 'armor' is false, signature: detached signature if 'detached' is true}