Enhance some bits of the suEXEC docco to be a bit more precise

in regard to suEXEC's docroot handling and it's preconditions;
see PR#21873 and #21874.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@100788 13f79535-47bb-0310-9956-ffa450edef68