Improving PHP-FPM documentation

[joshtrichards]

Using Nextcloud with PHP-FPM isn't that well documented. It's also not well documented officially (by PHP), but it's:

• widely used
• the only way to use Nginx with Nextcloud
• routinely perceived as "better performing" and, as a result, installed by users that lack experience with it and then blame Nextcloud for poor performance and service problems

Specifically we should explore ways to alert readers to:

• the defaults (e.g. pm.max_children and pm) and their impact on Nextcloud
• which values to focus on adjusting
• basic troubleshooting
• some possible external resources for additional general PHP-FPM guidance

Existing coverage:

• https://docs.nextcloud.com/server/latest/admin_manual/installation/server_tuning.html#tune-php-fpm

• https://docs.nextcloud.com/server/latest/admin_manual/installation/source_installation.html#php-fpm-configuration-notes

• https://docs.nextcloud.com/server/latest/admin_manual/installation/nginx.html

Sources of possible interest:

• https://medium.com/@mgonzalezbaile/demystifying-nginx-and-php-fpm-for-php-developers-bba548dd38f9