Where are the contents of /proc of the Linux kernel documented?

The documentation for Linux's implementation of /proc is in Documentation/filesystems/proc.txt in the kernel documentation.

Beware that /proc is one of the areas where *ixes differ most. It started out as a System V specific feature, was then greatly extended by Linux, and is now in the process of being deprecated by things like /sys. The BSDs — including OS X — haven't adopted it at all. Therefore, if you write a program or script that accesses things in /proc, there is a good chance it won't work on other *ixes.

Yes, there is a man page in section 5 "File formats and conventions":

man 5 proc

It is surprisingly long, though - it describes 189 kinds of files in /proc.