Update texts about multi-signature

Live preview:
http://bitcointest2.us.to/en/bitcoin-for-businesses
http://bitcointest2.us.to/en/secure-your-wallet

Some texts about multisig are a little outdated and inaccurate now (describe it as a soon-to-become-usable feature). I've tried to refresh these texts, and took the opportunity of improving them with more concise and relevant examples.