documenting --timestamp-old-table

Author: Shlomi Noach

doc/command-line-flags.md

@@ -130,3 +130,7 @@ See `approve-renamed-columns`
 ### test-on-replica
 
 Issue the migration on a replica; do not modify data on master. Useful for validating, testing and benchmarking. See [testing-on-replica](testing-on-replica.md)
+
+### timestamp-old-table
+
+Makes the _old_ table include a timestamp value. The _old_ table is what the original table is renamed to at the end of a successful migration. For example, if the table is `gh_ost_test`, then the _old_ table would normally be `_gh_ost_test_del`. With `--timestamp-old-table` it would be, for example, `_gh_ost_test_20170221103147_del`.