A number of fixes to the admin guide.

First, add missing reqDN indexing.
Second, convert most references to bdb/hdb to mdb, unless the section was specific to bdb/hdb.
Third, use "dc=example,dc=com" consistently throughout the example set

Update sections 6.2.2.1, 6.2.3.3

Ryan Tandy authored Jul 14, 2014 and Quanah Gibson-Mount committed Sep 21, 2015

In 2.4 syncrepl was changed to take its TLS defaults from the slapd
configuration (commit 1cc1f9b1) and the man page was updated to document
that (commit 27dd7a53). This updates the admin guide to match.

New Access Control Section, including a new Sets section incorporating ITS#5278, ITS#5279 and ITS#5281.

New Access Control Section, including a new Sets section incorporating ITS#5278, ITS#5279 and ITS#5281.