Default account clarifications

[nuke-web3]

New features (not yet ready) to bring up in docs when merged.
paritytech/substrate#10403
paritytech/substrate#10413
&&
paritytech/polkadot#4500
paritytech/cumulus#842

[shawntabrizi]

To be placed in the "dropbox" section of the docs.

covers:

• known private key for 0 address, specifically for sr25519/ed25519 (doesnt apply to ethereum or some of the other ones)
• warning not to use that address
• leads to no default implementaiton for address, and how to simply work around it with .unwrap_or([1u8; 32].into())