why:
Testing against a replay unit test for Devnet4 made it necessary to
adjust the TTD handling. Without updated, importing fails at block #5646
which is the parent of the terminal PoW block. Similar considerations
apply for Devnet5 and Kiln.
details:
For documentation, see comments in the file tx_pool.nim.
For prettified manual pages run 'make docs' in the nimbus directory and
point your web browser to the newly created 'docs' directory.