doc_functions.txt is a special text file that can be found in all Sphere releases. It lists and explains most functions and their parameters, and is one of the most used and most useful docs about the functions. It remains a relatively unformatted plain text file and previously attempts have been made to compile the same information into a more visually appealing format; this wiki is one of those attempts.
NOTE: Spherical is currently busy with the Functions page, which will contain all functions and more description per function. You can try to check out your function there first. Use this page if there was no extended documentation on your specific function.
The original documentation
Following: a dump of the original doc_functions.txt.