Undocumented implementation detail in _burn

[Cody Born]

In the current implementation, the minter can resurrect a burned NFT. This might be unexpected behavior for some, so mentioning this behavior in the notice section for _burn would be helpful.

[Tadej Vengust]

#208
@PhABC @codyborn Will this be sufficient?

[William Entriken]

Ping @codyborn @PhABC

[Philippe Castonguay]

I think that should be enough. Projects might want to "revive" tokens and nothing about the standard states that burning should be permanent.