Add some documentation on the ServerLimit and ThreadLimit

directives.

Both were added to mpm_common.html even though ThreadLimit
is used only by worker at the moment.  It is expected that
ThreadLimit will be supported by WinNT and perchild MPMs
in the future.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92536 13f79535-47bb-0310-9956-ffa450edef68