Address a couple of nits with recent En(De)crypt.
Address a couple of nits with recent En(De)crypt.

This commit adds comments about the specific crypto key types, moves the
selectCryptoKey function before the Encrypt/Decrypt functions that call it
to be more consistent with the rest of the code base, and slightly
modifies the verbiage of the comment.

Stash-Crypto/btcwallet,btcsuite/btcwallet,decred/dcrwallet,decred/dcrwallet,jrick/dcrwallet,Roasbeef/btcwallet,synrg-labs/btcwallet,jrick/btcwallet,btcsuite/btcwallet,Roasbeef/btcwallet,jrick/btcwallet,Stash-Crypto/btcwallet,synrg-labs/btcwallet,jrick/dcrwallet