If like the zoo you upgraded from debian 8 to debian 9 (my blog) then apache2-doc fails to do something on the lines of
ERROR: Conf apache2-doc does not exist!
dpkg: error processing package apache2 (–configure):
But apache still runs. – An purge and install eg:
apt purge apache2-doc;apt install apache2-doc
Fixes it so shit software like systemd will not complain about it. Honestly no idea why you have to do this when the version of the package was current but that’s how things go with systemd