Document .user.ini Files for Setting ini Values

**Is your feature request related to a problem? Please describe.**

In many cases PHP may not have access to a server's `php.ini` files or the ability to set env variables that PHP can see. They also may be working in a system where they're using one instance of PHP to manage different PHP applications.

**Describe the solution you'd like**
We should document the availability of user level configuration of php.ini values via [the `.user.ini` files for PHP-FPM](https://www.php.net/manual/en/configuration.file.per-user.php)

See also: https://discuss.elastic.co/t/how-to-configure-apm-agent-for-multiple-php-sites-on-one-machine/313508/4