Some .env parameters are undocumented

[gunchleoc]

I wanted to find out what PF_BOUNCER_ENABLED does, but it's not listed on https://docs.pixelfed.org/running-pixelfed/configuration.html

There might be others.

As an icing on the cake, documentation could be shown on /i/admin/diagnostics/home too.

Maybe it's possible to find a format so that the documentation can be generated from the code? Keeping it close to the code will make it easier not to forget documentation of new parameters.