Further cosmetic review of hashfn_unstable.h
In follow-up to
e97b672c8,
* Flesh out comments explaining the incremental interface
* Clarify detection of zero bytes when hashing aligned C strings
The latter was suggested and reviewed by Jeff Davis
Discussion: https://postgr.es/m/
48e8f8bbe0be9c789f98776c7438244ab7a7cc63.camel%40j-davis.com