docs: clean up documentation about decryption policies

docs: clean up documentation about decryption policies

Now that the range of sensible decryption policies has come into full
view, we take a bit of space to document the distinctions.

Most people will use either "auto" or "true" -- but we provide "false"
and "nostash" to handle use cases that might reasonably be requested.

Note also that these can be combined in sensible ways.  Like, if your
mail comes in regularly to a service that doesn't have access to your
secret keys, but does have access to your index, and you feel
comfortable adding selected encrypted messages to the index after
you've read them, you could stay in "auto" normally, and then when you
find yourself reading an indexable message (e.g. one you want to be
able to search for in the future, and that you don't mind exposing to
whatever entities have access to your inde), you can do:

    notmuch reindex --decrypt=true id:whatever@example.biz

That leaves your default the same (still "auto") but you get the
cleartext index and stashed session key benefits for that particular
message.