Add documentation for s2i_ASN1_INTEGER and related functions

Add documentation for s2i_ASN1_INTEGER and related functions

These functions convert strings to internal objects and vice versa.
This is a best effort, probably with a lot of room for improvement,
which can happen in tree if anyone cares. It's better than nothing.
Nothing in turn would be significantly better than the utter garbage
a related project has managed to land as part of their efforts towards
significant documentation improvements in a recent major relase.

This leaves a dangling reference to the misnamed X509V3_METHOD_get_nid(3)
which I may or may not fill in the future.

I am unsure about the HISTORY section's precision, but that's what I got
from cvs history. All these functions are about a quarter century old
(and it shows), so I don't think it matters very much.