Backported mod_authn_dbm documentation update. Corrected broken link and add...

Backported mod_authn_dbm documentation update. Corrected broken link and add an example about Berkeley db. PR: 59558

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1744186 13f79535-47bb-0310-9956-ffa450edef68