Fix the docs to match actual behavior of EINHORN_FD_N vars

They actually count from 0, not 1. I hadn't originally intended for
this to be the behavior, but now that the code is written, it seems
preferable to attempting to do a bunch of conversions between
0-indexing and 1-indexing, so I think I'd prefer to keep it.

@gdb Will merge if you don't have a strong preference for whether these should be 1-indexed or 0-indexed, but I think at this point better to keep code and fix docs than vice versa.