updateproxy, updatecluster check annexobjects=yes

updateproxy, updatecluster: Prevent using an exporttree=yes special remote
that does not have annexobjects=yes, since it will not work.

doc/git-annex-updateproxy.mdwn

@@ -28,6 +28,14 @@ a proxy.
 
 Proxies can only be accessed via ssh or by an annex+http url.
 
+To set up proxying to a special remote that is configured with
+exporttree=yes, its necessary for it to also be configured with
+annexobjects=yes. And, "remote.<name>.annex-tracking-branch" needs to
+be configured to the branch that will be exported to the special remote.
+When that branch is pushed to the proxy, it will update the tree exported
+to the special remote. When files are copied to the remote via the proxy,
+it will also update the exported tree.
+
 # OPTIONS
 
 * The [[git-annex-common-options]](1) can be used.
@@ -36,6 +44,7 @@ Proxies can only be accessed via ssh or by an annex+http url.
 
 * [[git-annex]](1)
 * [[git-annex-updatecluster]](1)
+* [[git-annex-export]](1)
 
 # AUTHOR
 

doc/todo/git-annex_proxies.mdwn

@@ -41,9 +41,6 @@ Planned schedule of work:
 	  export not supported
 	failed
 
-* Prevent `updateproxy` from enabling an exporttree=yes special remote
-  that does not have annexobjects=yes, to avoid foot shooting.
-
 * Handle cases where a single key is used by multiple files in the exported
   tree. Need to download from the special remote in order to export
   multiple copies to it. (In particular, this is needed when using
@@ -69,6 +66,10 @@ Planned schedule of work:
 * When getting from a P2P HTTP remote, prompt for credentials when
   required, instead of failing.
 
+* Prevent `updateproxy` and `updatecluster` from adding
+  an exporttree=yes special remote that does not have
+  annexobjects=yes, to avoid foot shooting.
+
 ## items deferred until later for p2p protocol over http
 
 * `git-annex p2phttp` should support serving several repositories at the same