formal:

- <em> -> <var>
- markup changes
- some whitespace reformatting

content:
- fix directive contexts
- clarify the flag directives and add default values
- add short description for CacheDefaultExpire


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@97418 13f79535-47bb-0310-9956-ffa450edef68