Edit RST admin guide for clarity and accuracy

Edit RST admin guide for clarity and accuracy

Some more notable changes include:

* plugin development is a top-level guide, so remove the advanced
  topic page in the admin guide.
* Remove krb5-appl references in firewall configuration section.
* Remove dns.rst since it was only referenced from a man page (which
  doesn't work) and just contained a pair of links, one of which was
  to the immediately preceding section.
* Don't document TXT record support as if it were supported in the
  default build.  It can be documented in an appendix.
* A couple of troubleshooting segments gave very
  specifically-applicable advice based on general error messages, or
  described problems which were fixed several releases ago.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25762 dc483132-0cff-0310-8789-dd5450dbe970