website: Clarify upgrade procedures. Fixes #187.

website/source/docs/compatibility.html.markdown

@@ -15,10 +15,10 @@ We promise that every subsequent release of Consul will remain backwards
 compatible with _at least_ one prior version. Concretely: version 0.5 can
 speak to 0.4 (and vice versa), but may not be able to speak to 0.1.
 
-The backwards compatibility must be explicitly enabled: Consul agents by
-default will speak the latest protocol, but can be configured to speak earlier
+The backwards compatibility is automatic unless otherwise noted. Consul agents by
+default will speak the latest protocol, but can understand earlier
 ones. If speaking an earlier protocol, _new features may not be available_.
-The ability for an agent to speak an earlier protocol is only so that they
+The ability for an agent to speak an earlier protocol is so that they
 can be upgraded without cluster disruption.
 
 This compatibility guarantee makes it possible to upgrade Consul agents one

website/source/docs/upgrading.html.markdown

@@ -13,10 +13,36 @@ keep in mind when using Consul.
 
 This page documents how to upgrade Consul when a new version is released.
 
-## Upgrading Consul
+## Standard Upgrades
 
-In short, upgrading Consul is a short series of easy steps. For the steps
-below, assume you're running version A of Consul, and then version B comes out.
+For upgrades we strive to ensure backwards compatibility. To support this,
+nodes gossip their protocol version and builds. This enables clients and
+servers to intelligently enable new features when available, or to gracefully
+fallback to a backward compatible mode of operation otherwise.
+
+For most upgrades, the process is simple. Assuming the current version of
+Consul is A, and version B is released.
+
+1. On each server, install version B of Consul.
+
+2. Shut down version A, restart with version B.
+
+3. Once all the servers are upgraded, begin a rollout of clients following
+   the same process.
+
+4. Done! You are now running the latest Consul agent. You can verify this
+   by running `consul members` to make sure all members have the latest
+   build and highest protocol version.
+
+
+## Backward Incompatible Upgrades
+
+In some cases, a backwards incompatible update may be released. This has not
+been an issue yet, but to support upgrades we support setting an explicit
+protocol version. This disables incompatible features and enables a 2-phase upgrade.
+
+For the steps below, assume you're running version A of Consul, and then
+version B comes out.
 
 1. On each node, install version B of Consul.