Document DevActivityPanel option

[norohind]

It took me some time in first place to figure out such option exists, I hope adding it to documentation is going to save somebody time.

[deluan]

Sorry for missing this. I don't want to start documenting the Dev* config options, as they are temporary and used mainly for testing features.

The issue about the UI freezing that DevActivityPanel "solves" is actually a workaround, that should be fixed in the new UI I'm working on. I'm ok with adding this workaround to a Troubleshooting section, or even the FAQ, but don't think it makes sense to add it to the list of documented config options.

[norohind]

I like the idea to add this at least to troubleshooting section. I will repurpose this PR for it in following couple of days then.