Document extensions to the qrexec policy daemon protocol

They were not documented, and several are not optional.

Fixes: QubesOS/qubes-issues#9038

doc/qrexec-policy-daemon.rst

@@ -30,4 +30,16 @@ Any possible extensions may be placed on next lines.
 All responses that do not start with `result=allow` or `result=deny` are
 incorrect and will be rejected.
 
-End of response and request is always an empty line.
+End of request is always an empty line.
+Response is always terminated by EOF.
+
+Extensions include:
+
+- `target=`: Name of the target, optionally preceded by `@dispvm:`
+  `@dispvm:` prefix means that this is a disposable VM template and a new disposable VM will be created automatically.
+  In allow responses, ignored if `target_uuid=` is present, required otherwise.
+  Forbidden in deny responses.
+- `autostart=`: `True` to automatically start the VM, `False` to not start it.
+  Anything else is invalid.
+  Required in allow responses, forbidden in deny responses.
+- `requested_target=`: Normalized version of the target domain.