revlog: document that mmap resources are released implicitly by GC

It's okay-ish, but currently the open fd and the mapping itself are leaked
until the indexdata is deallocated. If revlog had close(), the underlying
resources should be closed there as well, but AFAIK there's no such hook
point.

mercurial/revlog.py

@@ -440,6 +440,8 b' class revlog(object):'
             with self._indexfp() as f:
                 if (mmapindexthreshold is not None and
                     self.opener.fstat(f).st_size >= mmapindexthreshold):
+                    # TODO: should .close() to release resources without
+                    # relying on Python GC
                     indexdata = util.buffer(util.mmapread(f))
                 else:
                     indexdata = f.read()