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 to note that MDB is the primary backend and HDB/BDB are
deprecated.

Commit 279594c2 officially marked
slapd-mdb as being the recommended backend for OpenLDAP, superseding
back-hdb.  This man page was missed during that update.

Fix repeated use of the word is

Howard Chu authored 7 years ago and Quanah Gibson-Mount committed 7 years ago

Changed as of ITS#4327 commit 86d10729

ITS#8121 - Note ldap_sasl_bind and ldap_sasl_bind_s can be used to make simple binds via the LDAP_SASL_SIMPLE mechanism

ITS#8635 - Note that non-zero serverID's are required for MMR, and that serverID 0 is specific to single master replication only

Update sections 6.2.2.1, 6.2.3.3