update stale documentation on Mapper class
update stale documentation on Mapper class

don't get into details here that have changed,
point to the main narrative documentation instead.

Change-Id: I5bc0829f6ff282ca104f18deb9ec9857b6e1c8ac
(cherry picked from commit 9298ce03e1181d5bc00c1891663433606ead1223)

j5int/sqlalchemy,j5int/sqlalchemy