expand third common in the expression functions doc

Explain some risks at looking at per-request environment variables
set by other modules within <If>.

http://stackoverflow.com/questions/39462011/htaccess-if-else-always-selects-else




git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1761272 13f79535-47bb-0310-9956-ffa450edef68