doc/admin: update tracing docs

Created by: bobheadxi

Wording tweaks and fixes. I also finally properly read this "Jaeger debugging algorithm" which isn't actually a Jaeger configuration thing but a general "how to use traces" guide! So I've gone ahead and rewritten it as a "how to use traces" guide. Also adds a note that selective still outputs a lot of traces in the background

Test plan

sg run docsite