Explain when the surrounding double quotes arround the expr argument MUST be...

Explain when the surrounding double quotes arround the expr argument MUST be avoided. (part of r1829390 in trunk)

Remove trailing spaces to synch with trunk.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1829392 13f79535-47bb-0310-9956-ffa450edef68