Clarify `createemptyblocks` transition in Clique

[joaniekube]

Clarify this section to reflect changes for Clique createemptyblocks transition made in PR #6608

Context: Previously the createemptyblocks option, introduced in besu-eth/besu#6082, prevented syncing a network that had some empty blocks at the beginning - This occurred if the network started with the createemptyblocks option enabled or not set, and later, the setting was switched to false.

See the following for more information:

• Clique createemptyblocks transition besu#6608