email: note that passwordCommand should output '\n'

This is because some programs, for example msmtp, expect the output to
end with a newline character.

modules/accounts/email.nix

@@ -182,8 +182,8 @@ let
         apply = p: if isString p then splitString " " p else p;
         example = "secret-tool lookup email me@example.org";
         description = ''
-          A command, which when run writes the account password on
-          standard output.
+          A command that writes the account password on standard
+          output followed by a newline when it is run.
         '';
       };