Document Substrate's Nonce Behaviour

[HCastano]

Substrate is unusual from other blockchains because an account nonce can be reset if, for
example, an account's balance drops below the existential deposit for the Substrate chain
it's on.

In other blockchain ecosystems this is not the case, so an account nonce is typically
used to provide replay protection. In Substrate this is done through the use of
transaction mortality.

Since smart contract developers will be coming from different ecosystems we should
document this behaviour for them, and have some possible mitigation.

Here are some resources to help get these docs started:

• Original Substrate issue
• Discussion about UX improvements
• Polkadot transaction mortality docs