Document quilt-server-launch.properties and quilt-server-launcher.properties

[clarfonthey]

This has always been extremely opaque for Fabric, and it would be nice to have some central documentation for Quilt on this file. I personally use it to modify the serverJar property so I can move the quilt-server-launch.jar to a common location for multiple servers.

It appears that these two files exist probably by mistake, since I'm assuming that they're supposed to be the same file, although they've probably been separate on Fabric for who knows how long.