1404c57403 [doc] Coin: explain that IsSpent() can also mean never existed (Sjors Provoost)
Pull request description:
This can be especially confusing where `AccessCoin()` is used with logic like this:
```c++
while (iter.n < MAX_OUTPUTS_PER_BLOCK) {
const Coin& alternate = view.AccessCoin(iter);
if (!alternate.IsSpent()) return alternate;
```
ACKs for top commit:
practicalswift:
ACK 1404c57403
MarcoFalke:
ACK 1404c57403
jnewbery:
utACK 1404c57403
Tree-SHA512: 418618dd7e08bd5cc8360e3501d0f57e34100e5101ad3b8e0a819923fa860f44c7f2fada0f8447a1af3c2601fd72bfe619b91ff2f26f7133ceaeb0c98b017b12
55ceaeb8c4