Added a reference to how ListenCoresBucketsRatio influences the...

Added a reference to how ListenCoresBucketsRatio influences the Min|MaxSpareThreads lower bounds calculation in the docs.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1753074 13f79535-47bb-0310-9956-ffa450edef68