add --cache-last-block flag description

[ahamlat]

The flag --cache-last-block created in this PR hyperledger/besu#6009 is not yet documented.
This flags makes Besu cache n blocks data (block headers, block bodies, transactions' receipts and total difficulty).

The default number of blocks to keep in the cache is 0.

This PR will mainly improve the performances of some RPC calls :

Eth_getBlockByNumber
Eth_getBlockByHash
Eth_feeHistory
Eth_getTransactionReceipt
...
The biggest improvement is on Eth_feeHistory.

The new cache in DefaultBlockchain uses 1.35 GiB on Mainnet for 2048 last blocks :

Transactions receipts : 676 MiB
Blocks' bodies : 555 MiB
Blocks' headers : 2 MiB
Total difficulty : less than 1 MiB