Fix case in directive name, so that quickreference.xsl can extract default value correctly.

Add a note in ThreadsPerChild about the relationship with ThreadsLimit

r1838958, r1853617 and r1853618 in trunk

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1853619 13f79535-47bb-0310-9956-ffa450edef68