pveum: better LDAP/AD explanation + restructuring

pveum: better LDAP/AD explanation + restructuring

Begin "Authentication Realms" section with brief introduction to each
realm (this was there already, but inconsistent in the level of detail
given for each part).

Give each realm type its own subsection

Adds some more information on LDAP and Active Directory configuration.

Restructure the document to bring the "Syncing LDAP-Based Realms" section
closer to the LDAP and AD sections.

Explain LDAP parameters/options individually in list format

Remove some non-relevant information such as how to add system users

Some more minor langauge and formatting fixup