[PHP] improve SDK env var docs

[brettmc]

Could you add a bit more detail in the descriptions as to what users can accomplish by fiddling with these? In a separate PR, if you prefer.

Originally posted by @theletterf in https://github.com/open-telemetry/opentelemetry.io/pull/3952#discussion_r1481173745

In reference to the list of PHP-specific environment variables in the php SDK documentation.