Document new plugin configuration option

[bgravenorst]

Added a new plugin config option that controls how much time plugins can use to propose their own transactions during block creation.

The max time is configurable in term of percentage of the overall block creation max time, using the new configuration option plugin-block-txs-selection-max-time that by default is set to 50 meaning 50% or half of the overall block creation time.

For more information, see: hyperledger/besu#9184