Update account deletion requirements

[mDuo13]

The account deletion requirement docs need to be updated.

See DeleteAccount.cpp for the actual implemented requirements:

• An account cannot be deleted if it is the issuer of any NFTs in the ledger (it's OK if all its issued NFTs have been burned; you can calculate this with MintedNFTokens vs BurnedNFTokens in the account info)
• An account can only be deleted if FirstNFTSequence + MintedNFTokens + 256 is less than the current ledger index. (Added by fixNFTokenRemint amendment)
• Several new types of ledger entry are also deletion blockers. The account_objects API has a list of blocking types; this is hopefully consistent with AccountDelete, but the actual transactor is implemented with a list of non-blocking types. The documentation should also, ideally, mention which amendment adds/added each blocking type.

Ideally, the account deletion documentation should probably also have a list, table, or other guide with instructions on if & how you can clear each type of deletion blocker. (For example, with trust lines you can use an offer or a payment to reduce your balance to 0 if you are the holder, and then use TrustSet to return your limit & other settings to the default. An issuer would have to use clawback, if they have it enabled, to return the balance to 0. And if the other party of the trust line has any non-default settings, you cannot remove it.)