Add Warning about use of delete

[lucasAlonso]

Fixes #3400

Added Warning about delete operator on structs like EnumerableSet and EnumerableMap even though they contain a mapping that will not be cleared, and the operation will in fact corrupt the struct's data.

According to ethereum/solidity#11843 this operation will no longer be allowed in Solidity 0.9, but in the meantime we should document that delete should not be used with these types.

as user frangio stated, i checked the info and link

PR Checklist

• Documentation
  check if this Warning is useful and apply.
  Need any further warning or issue?

[Amxx]

Hello @lucasAlonso

While I do agree with the need for a warning, I'm not sure this is the right place to put it.

IMO this should be at the level of the solidity library (top part of the file).

[lucasAlonso]

Hi Amxx,
at the issue opened (#3400), @SamuelOsewa said
"I believe that it should be when the delete is used so if someone wants to see the use case they would see the comment as people tend to skip over the introduction."
Maybe two warnings? or add a more general issue that involves change delete for clear
This is my first PR, so sorry if I'm overthinking :)

[Amxx]

It is important for this warning to show up in the documentation. For that to happen, it must be part of the natspec comments.

[lucasAlonso]

Amxx changed Warning placement, now at top part of the file.
Thxs!

[Amxx]

Format is not correct.

See utils/Address.sol for how to properly format a warning.

[Amxx]

Also, delete in not a "function", and there is no clear function in this structure.

I would suggest something like:

Trying to delete such a structure from storage will likely result in data corruption, rendering the structure unusable.
See ethereum/solidity#11843 for more info

In order to clean an EnumerableMap, you can either remove all elements one by one or create a fresh instance using an array of EnumerableMap.

[lucasAlonso]

Changed wording and format.
maybe add an issue at contracts/utils/cryptography/MerkleProof.sol to make Warning's Format consistent ?