State the Held Poll's Immediate 44 in scripts/README.md and check_nodes's Node Keys #2725

Two documentation gaps around scripts/pr_review.py wait's held-head check poll, raised in local review on #2722 and left there once that pull request's edit budget was spent.

• scripts/README.md's 30 paragraph lists what holds a held poll to its timeout, and the attest paragraph points at it as the full list. It does not say that on a BLOCKED merge a stuck required check (failed, not picked up, or never posted) ends the poll at once with 44, even while another required check still runs. The wait help text's 44 entry states it.
• check_nodes's docstring still describes each node as {name, state, conclusion, since}. Nodes now carry a required key that held_checks_open decides on, and an unreadable member carries an unreadable marker, which the docstring already left out.

Done looks like

• The README names the immediate 44 beside the held 30 conditions.
• The check_nodes docstring names every key a node can carry.