docs(checker): a fence suspends the run, and only a runnable fence is exempt

Two surprises reported by agents in the same hour, neither a behaviour change.

A fence SUSPENDS the count rather than ending the run, so two prose lines above
a fence and two below is a run of four. That is deliberate -- a reader meets it
as one uninterrupted block -- but an agent capped a file to what looked like
2 and 2 and the checker correctly called it 5.

And rustdoc does not execute ```text, ```ignore or ```no_run. Two of the six
fences in this tree were ```text, so they were claiming the doctest exemption
for prose. Only a runnable fence earns it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/check-comment-runs.py

@@ -6,8 +6,17 @@ A comment run is at most two lines, optionally naming a context/ file for the
 rest. See context/code/ for where the prose went, and improvise-9c8c for why.
 
 Counts `///`, `//!` and `//` alike. Attributes are not comments and are
-ignored. A ``` fence inside a doc comment suspends the count, because a
-doctest is a running test rather than prose -- see --list-doctests.
+ignored.
+
+A ``` fence SUSPENDS the count rather than ending the run: the fenced body is
+exempt (a doctest is a running test, not prose) but the prose on either side
+belongs to ONE run and is summed. Two lines above a fence and two below is a
+run of four, which is over the limit -- and that is deliberate, because a
+reader meets it as one uninterrupted comment block. See --list-doctests.
+
+Note rustdoc executes a bare ``` or ```rust fence and does NOT execute
+```text, ```ignore or ```no_run. Only a runnable fence earns the exemption;
+the rest is prose wearing a fence and the cap applies to it.
 """
 
 import argparse