notes on ImportableContents history not being used for retrieval

Types/Import.hs

@@ -59,6 +59,12 @@ data ImportableContents info = ImportableContents
 	-- ^ Used by remotes that support importing historical versions of
 	-- files that are stored in them. This is equivilant to a git
 	-- commit history.
+	--
+	-- When retrieving a historical version of a file,
+	-- old ImportLocations from importableHistory are not used;
+	-- the content is no longer expected to be present at those
+	-- locations. So, if a remote does not support Key/Value access,
+	-- it should not populate the importableHistory.
 	}
 	deriving (Show, Generic)
 

doc/design/external_special_remote_protocol/export_and_import_appendix.mdwn

@@ -168,6 +168,8 @@ support a request, it can reply with `UNSUPPORTED-REQUEST`.
     block of responses. This can be repeated any number of times
     (indicating a branching history), and histories can also
     be nested multiple levels deep.  
+    This should only be used when the remote supports using
+    "TRANSFER RECEIVE Key" to retrieve historical versions of files.
   * `END`  
     Indicates the end of a block of responses.
 * `LOCATION Name`