Document requirement for prefork MPM when using mod_php

[ghost]

This is a little bit hidden in:

https://github.com/owncloud/documentation/blob/3a5289e71a70d7b53b27cadd61804db5788ba9c4/admin_manual/configuration/server/oc_server_tuning.rst#mpm

but is a hard requirement when using mod_php because that module is not thread safe.

https://secure.php.net/manual/en/install.unix.apache2.php

Debian e.g. is enforcing this worker in their libapache2-mod-php7.0.postinst script:

apache2_msg "err" "$DPKG_MAINTSCRIPT_PACKAGE: Could not switch to prefork MPM, not enabling PHP 7.0"

but there are other distros / systems where people still enable multi-threaded MPMs like seen in e.g.:

https://central.owncloud.org/t/many-operation-canceled-connection-closed-errors-during-sync/6006/7

In general using anything else then prefork (like worker or event) will cause issues like the known:

Connection closed
Operation canceled

and maybe also the "expected filesize 4734206 got 458752" messages described by some one in here:

https://central.owncloud.org/t/expected-filesize-xxx-got-yyy-0/816

Edit

Some more refs: owncloud/core#22957, owncloud/core#12193, owncloud/core#26406, owncloud/core#9832, owncloud/core#9909, owncloud/core#28888

Edit2

I have also moved the MPM part from the OC Server Tuning docs. This is not really a tuning recommendation but a installation requirement. Furthermore added a few additional references to the troubleshooting guide.

[ghost]

Btw. maybe some one with an account on central could point the users in https://central.owncloud.org/t/many-operation-canceled-connection-closed-errors-during-sync/6006 to this pull request and let them know that using worker instead of event will still cause issues.

[settermjd]

LGTM. Thanks for contributing the changes.