spec: Document our circuit close reasons

In or.h, we define the reason to close a circuit:

/* Reasons why we (or a remote OR) might close a circuit. See tor-spec.txt for
 * documentation of these. */
#define END_CIRC_REASON_MIN_            0
#define END_CIRC_REASON_NONE            0
#define END_CIRC_REASON_TORPROTOCOL     1
#define END_CIRC_REASON_INTERNAL        2
#define END_CIRC_REASON_REQUESTED       3
...

Even though it says "See tor-spec.txt", those values aren't defined at all in tor-spec.txt, only the stream reasons are.

This is a bit annoying because these values don't have any documentation on why and when they should be used.