Update some comments for fasthash

Update some comments for fasthash

- Add advice about hashing multiple inputs with the incremental API
- Generalize statements that were specific to C strings to include
  all variable length inputs, where applicable.
- Update comments about the standalone functions and make it easy to
  find them.

Reported-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: zengman <zengman@halodbtech.com>
Discussion: https://postgr.es/m/CANWCAZZgKnf8dNOd_w03n88NqOfmMnMv2=D8_Oy6ADGyiMq+cg@mail.gmail.com
Discussion: https://postgr.es/m/CANWCAZa-2mEUY27xBw2TpsybpvVu3Ez4ABrHCBqZpAs_UDTj2Q@mail.gmail.com