[docs] explain what you mean by "arbitrary precision"

[emilyyyylime]

When I first read fend supports arbitrary-precision arithmetic, I was under the impression that means I can write something like sqrt 2 to 1000 dp and receive √2 to 1000 decimal places (even if extremely slowly). I now understand that the arbitrary precision of fend is really more of an 'arbitrary size' as functions like sqrt cannot yield arbitrarily precise results. I'd appreciate if this could be explained more prominently in the docs & README, perhaps with a simple example—to avoid misleading others in the future