docs/system: clarify deprecation schedule

The sentence explaining the deprecation schedule is ambiguous. Make it
clear that a feature deprecated in the Nth release is guaranteed to
remain available in the N+1th release. Removal can occur in the N+2nd
release or later.

As an example of this in action, see commit
25956af ("block: Finish deprecation of
'qemu-img convert -n -o'"). The feature was deprecated in QEMU 4.2.0. It
was present in the 5.0.0 release and removed in the 5.1.0 release.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200915150734.711426-1-stefanha@redhat.com>

docs/system/deprecated.rst

@@ -3,10 +3,11 @@ Deprecated features
 
 In general features are intended to be supported indefinitely once
 introduced into QEMU. In the event that a feature needs to be removed,
-it will be listed in this section. The feature will remain functional
-for 2 releases prior to actual removal. Deprecated features may also
-generate warnings on the console when QEMU starts up, or if activated
-via a monitor command, however, this is not a mandatory requirement.
+it will be listed in this section. The feature will remain functional for the
+release in which it was deprecated and one further release. After these two
+releases, the feature is liable to be removed. Deprecated features may also
+generate warnings on the console when QEMU starts up, or if activated via a
+monitor command, however, this is not a mandatory requirement.
 
 Prior to the 2.10.0 release there was no official policy on how
 long features would be deprecated prior to their removal, nor