Documentation/technical/reftable: document rename in reflog.

Change-Id: I0fe7d28a772b1ee9eefd9a38bff5e08a8559988f
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>

Documentation/technical/reftable.md

@@ -566,6 +566,10 @@ supplied for the update.
 
 [update-ref]: https://git-scm.com/docs/git-update-ref#_logging_updates
 
+Contrary to traditional reflog (which is a file), renames are encoded as a
+combination of ref deletion and ref creation.
+
+
 #### Reading the log
 
 Readers accessing the log must first read the footer (below) to