Illustrate an ->iterator() implementation for mod_socache_shmcb.

Clarify that the iterator aught will make the users' life easier
by appending trailing \0 null chars.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@907937 13f79535-47bb-0310-9956-ffa450edef68