Fix typo (s/ProxyExpressDBMFile/ProxyExpressDBMType/)

PR 63119.

Move compatibility note in the module synopsis instead of repeating it in all directives.
Fix the way <default> values are given, so that they can be reflected in quickreference.html.
Fix the style used in <syntax> to be consistent with the documentation.
Improve syntax highlight.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1852270 13f79535-47bb-0310-9956-ffa450edef68