#
# The default processing model (MPM) is the process-based
# 'prefork' model.  A thread-based model, 'worker', is also
# available, but does not work with some modules (such as PHP).
# The service must be stopped before changing this variable.
#
#HTTPD=/usr/sbin/httpd.worker


Searching to switch to worker mode, I notice than this comment is still present but have no effect (keeping old behavior have been great, don't know if $HTTP in ExecStart line will work)


I think this comment should be removed.
Perhaps a new comment (for those used to change this) about how to switch to worker mode, which is, I think:

systemctl stop httpd.service
systemctl disable httpd.service
systemctl enable httpd-worker.service
systemctl start httpd-worker.service


Really a "minor" documentation bug