docs(knowledge): name the guard for the verification filter's two dialects

The location filter's section comment says THE TWO MUST CHANGE TOGETHER and
names tests/test_retrieval_scopes.py as what enforces it. The drift-check
filter added in #2086 has the same two-dialect structure and the same
hazard — a predicate applied in only one arm makes a record findable one
way and invisible another — but pointed at no guard, so the next person
had to discover that tests/test_snippet_drift_check.py walks both.

Also names the case that motivated `attention` existing at all: an ok
verdict whose code_sha has gone stale is neither `drifted` nor
`unverified`, and is the one shape a reader is likely to think redundant
and remove.

Written while verifying the plugin fixes end-to-end — this edit is what
the write-path trigger fired on, correctly surfacing snippet #2192 as
prior art at this exact path.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

src/scribe/services/knowledge.py

@@ -120,6 +120,12 @@ def _location_clause(parts: dict[str, str]):
 # the verdict's: jsonpath compares the two fields within the row, so this stays
 # one index-served predicate rather than a post-filter that would make the
 # pagination total a lie.
+#
+# Same two-dialect discipline as the location filter above, and the same hazard:
+# THE TWO MUST CHANGE TOGETHER. tests/test_snippet_drift_check.py is the guard —
+# it walks both dialects over the same cases, including the one that motivated
+# `attention` existing (an ok verdict whose code_sha has gone stale, which is
+# neither `drifted` nor `unverified` yet plainly needs looking at).
 
 def verification_matches(data: dict | None, value: str) -> bool:
     """Python dialect of the verification predicate.