Improve LDAP admin documentation

Improve LDAP admin documentation

Use dc=example,dc=com as the example base DN instead of more archaic
forms.  Provide a little more cross-referencing of concepts and
mechanisms.  Add additional steps in the OpenLDAP setup instructions
for choosing DNs for the Kerberos container, KDC service, and kadmin
service.  Explain a little bit about what the Kerberos container and
realm container are.  Be clearer that using separate subtrees from the
realm container for principals is an option, not a necessity, and
don't use the base DN as an example of a separate subtree (it's
confusing).

ticket: 6418
target_version: 1.7
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@22088 dc483132-0cff-0310-8789-dd5450dbe970