improve docs about removeExportDirectory

Remote/Directory.hs

@@ -81,6 +81,8 @@ gen r u c gc = do
 				, retrieveExportWithContentIdentifier = retrieveExportWithContentIdentifierM dir
 				, storeExportWithContentIdentifier = storeExportWithContentIdentifierM dir
 				, removeExportWithContentIdentifier = removeExportWithContentIdentifierM dir
+				-- Not needed because removeExportWithContentIdentifier
+				-- auto-removes empty directories.
 				, removeExportDirectoryWhenEmpty = Nothing
 				, checkPresentExportWithContentIdentifier = checkPresentExportWithContentIdentifierM dir
 				}

doc/design/external_special_remote_protocol.mdwn

@@ -198,8 +198,8 @@ replying with `UNSUPPORTED-REQUEST` is acceptable.
   respond with `REMOVE-SUCCESS`.
 * `REMOVEEXPORTDIRECTORY Directory`  
   Requests the remote remove an exported directory.  
-  If the remote does not use directories, or automatically cleans up
-  empty directories, this does not need to be implemented.  
+  If the remote does not use directories, or REMOVEEXPORT cleans up
+  directories that are empty, this does not need to be implemented.  
   The directory will be in the form of a relative path, and may contain path
   separators, whitespace, and other special characters.  
   Typically the directory will be empty, but it could possibly contain

doc/design/importing_trees_from_special_remotes.mdwn

@@ -281,7 +281,9 @@ the race.
 
 removeExportDirectoryWhenEmpty is used instead of removeExportDirectory.
 It should only remove empty directories, and succeeds if there are files
-in the directory.
+in the directory. This is needed because removeExportDirectory removes
+non-empty directories, but when importing the stuff in those directories is
+stuff that ought to be imported.
 
 checkPresentExportWithContentIdentifier is used instead of
 checkPresentExport. It should verify that one of the provided