spec: Emphasise the capability for revocation of earlier alerts

Modelling carried out for NHSX ("Effective Configurations of a Digital
Contact Tracing App: A report to NHSX") suggests that an important
factor in the effectiveness of any contact tracing approach is the
ability to provide fast alerts of self-reported symptoms combined with
the ability to subsequently revoke the alert if a test proves
negative.

This is already supported by the CX protocol, since an alertLevel will
supersede any earlier reported alertLevel and there is already a
defined alertLevel of no-alert(0).  Emphasise this ability within the
cover page bullet points (replacing the point about immediate
deployability, since there is no more space on the cover page).

Signed-off-by: Michael Brown <[email protected]>

README.md

@@ -41,14 +41,14 @@ The architecture provides the following features:
     ability for users to distinguish between self-reported symptoms and
     medical test results.
 
+  - **Revocable diagnoses**: a negative medical test result can
+    supersede initial self-reported symptoms, allowing alerted users to
+    minimise unnecessary time spent in isolation.
+
   - **Distributed contact identification**: users can identify only
     their own hazardous contacts, and no central authority has access to
     the information required to identify contacts.
 
-  - **Immediate deployability**: the protocol may be implemented as an
-    app on existing Android and iOS devices, with no requirement for
-    changes to the core operating system.
-
   - **Interoperability**: the protocol may be implemented by any device
     and as part of any app, with all apps and all devices able to
     benefit equally from the same distributed data set.

spec/cx.lyx

@@ -301,19 +301,19 @@ ted symptoms and medical test results.
 \begin_layout Itemize
 
 \series bold
-Distributed contact identification
+Revocable diagnoses
 \series default
-: users can identify only their own hazardous contacts, and no central authority
- has access to the information required to identify contacts.
+: a negative medical test result can supersede initial self-reported symptoms,
+ allowing alerted users to minimise unnecessary time spent in isolation.
 \end_layout
 
 \begin_layout Itemize
 
 \series bold
-Immediate deployability
+Distributed contact identification
 \series default
-: the protocol may be implemented as an app on existing Android and iOS
- devices, with no requirement for changes to the core operating system.
+: users can identify only their own hazardous contacts, and no central authority
+ has access to the information required to identify contacts.
 \end_layout
 
 \begin_layout Itemize