hash/maphash: adjust package comment

Add note about using per-use seeds.

Delete "collision-resistant but" in:
> The hash functions are collision-resistant but not cryptographically secure.

"Collision-resistant" has a precise cryptographic meaning that is
incompatible with "not cryptographically secure".
All that is really meant by it here here is "it's a good hash function",
which should be established already.

Also delete:
> The hash value of a given byte sequence is consistent within a
> single process, but will be different in different processes.

This was added for its final clause in response to #37040,
but "The hash value of a given byte sequence" is by design not a
concept in this package. Only "... of a given seed and byte sequence".
And seeds cannot be shared between processes, so again by design
you can't even set up the appropriate first half of the sentence
to say the second half.

Change-Id: I2c02bee0e804ef3b120cb4752bf89e60f3f5ff5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/255968


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>