mdbx: backport - Mention MDB_PREV_MULTIPLE along with MDB_NEXT_MULTIPLE.
mdbx: backport - Mention MDB_PREV_MULTIPLE along with MDB_NEXT_MULTIPLE.

Change-Id: I0c216203c3aa2005ef254293c1c472c9b7f257f3

leo-yuriev/libfpta,leo-yuriev/libfpta