hash: document that the encoded state may contain input in plaintext

hash: document that the encoded state may contain input in plaintext

The cryptographic checksums operate in blocks of 64 or 128 bytes,
which means that the last 128 bytes or so of the input may be encoded
in its original (plaintext) form as part of the state.
Document this so users do not falsely assume that the encoded state
carries no reversible information about the input.

Change-Id: I823dbb87867bf0a77aa20f6ed7a615dbedab3715
Reviewed-on: https://go-review.googlesource.com/77372
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>