Emphasize that the output of #printenv is plain text so that it doesn't dump a...

Emphasize that the output of #printenv is plain text so that it doesn't dump a bunch of unformatted text in an HTML doc.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1437833 13f79535-47bb-0310-9956-ffa450edef68