update AA key requirements

[Issue 10045 – Missed warning with wrong toHash() + toHash documentation](https://issues.dlang.org/show_bug.cgi?id=10045)
I'm sure it was. We stopped using `hash_t` ages ago. It's still there for backwards compatibility (and all it ever was was an alias), but it should probably be deprecated. Regardless, it shouldn't be in the documentation.