Clarify documentation about file+noindex repos

doc/config.rst

@@ -198,7 +198,9 @@ repository.
 
 ``cabal`` will construct the index automatically from the
 ``package-name-version.tar.gz`` files in the directory, and will use optional
-corresponding ``package-name-version.cabal`` files as new revisions.
+corresponding ``package-name-version.cabal`` files as new revisions. Note that
+Windows should use three slashes too, as in
+``file+noindex:///C:/absolute/path/to/directory``.
 
 For example, if ``/absolute/path/to/directory`` looks like
 ::