bug#45178: buffer-hash docstring should explain difference to secure-hash

[Stefan Kangas]

"Basil L. Contovounesios" <contovob@tcd.ie> writes:

> Lars Ingebrigtsen <larsi@gnus.org> writes:
>
>> `buffer-hash' is fast, so you can use it to compare the buffer contents
>> before and after doing things like `indent-sexp' to see whether that
>> changed anything.
>
> [I get that this is just an example, but in this case wouldn't you use
>  buffer-modified-tick or buffer-chars-modified-tick instead?]

AFAIU, this is used to detect when a command changed a buffer but the
buffer still is the same before and after the changes.  In that case, we
don't want it to be marked as changed even if there were changes taking
place in the background.  For example, see `fill-paragraph'.