crypto, hash: document marshal/unmarshal implementation

Unless you go back and read the hash package documentation, it's
not clear that all the hash packages implement marshaling and
unmarshaling. Document the behaviour specifically in each package
that implements it as it this is hidden behaviour and easy to miss.

Change-Id: Id9d3508909362f1a3e53872d0319298359e50a94
Reviewed-on: https://go-review.googlesource.com/77251
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

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