math/mp-sqrt.c: Explain the algorithm, and particularly the end condition.
The gnomic remark that `Increasing x is pointless when -q < 2 x + 1' was
enough to lead me back on the right lines, but is hardly adequate. I
ended up wasting quite a lot of time with a whiteboard, because the
`... + 1' makes the termination condition look just enough like `the
fraction comes out zero'. But no: that's (not quite) a
coincidence. (Thinking more carefully, it's no surprise that the delta
looks similar to the derivative, but it's definitely the former we're
interested in here, rather than the latter.)
~mdw / catacomb / commit