Some serious updates to the top section of the mod_dav docs. Many of the ideas

Some serious updates to the top section of the mod_dav docs.  Many of the ideas
are stolen from Greg's mod_dav site.  Some specifics:

- Change recommendations about where to place the dav lockfile and be specific
about required permissions.

- Remote the HEAD from the <Limit> block, since it is implied by GET.

- Add a security section discussing file permissions, auth issues, denial of
service, etc.

- Add a "complex config" section discussing the dynamic content issue.

Review requested.  Unfortunately, I can't check in the html because of
transformation problems in my setup.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@99469 13f79535-47bb-0310-9956-ffa450edef68