Some years back I decided to play around with FVWM. I was astounded with quality the man pages. FVWM isn't so much a window manager as it is a window manager *kit*, with lots & lots of configuration options. But the documentation is some of the best I've seen.
I've just been trying to work with lightdm here, myself (disable guest login & not auto-fill-in the last user name) and found the same as you. The config files have even been moved and no-one bothered to mention that.
Anyway, I usually end up in some user forum or other. Luckily I haven't had many unique problems. Almost always someone, somewhere, has had to figure it out before me & is willing to pass on the info.