The best (readable by mere mortals) document I found on sysfs is this tutorial by Greg Kroah-Hartman. The tutorial shows how to implement a driver using the sysfs interface. I did not find any references to standards or conventions about documenting valid values.
It seems to be so that any driver developer can implement things the way he or she likes. Finding the valid values for a sysfs entry is then dependend on the documentation given. Unfortunately there also seems to be no standard place to find the documentation.
You could search the linux source code (here for example). Unfortunately the linux kernel, being programmed in c, is riddled with macros and pointers, so one can not do a simple grep through the source.