fleetd #639: redact()'s comment claimed more than the code does

The paragraph added in d7f94ca ended with "This needs no knowledge of the key's
name and so protects a block scalar under any masked key, present or future."
The continuation masking is real and it is an improvement, but that sentence is
too strong: the masking only holds while the masked key's own line is inside the
hunk being printed.

redact() is fed `diff -u` output, which prints three lines of context. A block
scalar's body therefore often arrives with its key line left out. With no key
line, `masked` is never set and the body prints in full, with no "<redacted>"
anywhere. A blank line inside a block scalar loses the anchor the same way: a
blank diff line measures as indent 0, so `indent > masked_indent` is false and
the mask ends early — this time directly under a "<redacted>" marker.

Both were reproduced through the real script with --dry-run, each with a
positive control run first to prove the secret's lines actually reached the
output (without that control, "the secret never entered the diff" and "it
entered and was redacted" are indistinguishable). Filed as fleetd #639, which
also records that this is latent rather than live: today's fleetd.yaml holds 5
block scalars and all 5 sit under non-secret keys.

Comment text only. No change to redact() or to any other function, and no
change to the test suite. scripts/test-config-edit.sh still passes in a clean
copy (16 criteria + 3 extras, exit 0); bash -n clean.

The reason this is worth its own commit: #635 exists because an incomplete
redactor that looks complete is worse than one that visibly does nothing. A
comment that overstates the guarantee is the same defect in prose, and the next
session to read it has no other source.

scripts/config-edit.sh

@@ -159,7 +159,13 @@ done
 # The fix is structural, not another name to match: once a key line is masked, every following
 # line indented STRICTLY DEEPER than that key is masked too, by indentation alone, until the
 # indentation returns to the key's own level or shallower. This needs no knowledge of the key's
-# name and so protects a block scalar under any masked key, present or future.
+# name, so it covers a block scalar under any masked key — but ONLY while that key's own line is
+# itself inside the hunk being printed. `diff -u` prints just three lines of context, so a block
+# scalar's body often reaches this function with its key line left out; there is then nothing to
+# anchor to, `masked` is never set, and the body prints in full. A blank line inside a block
+# scalar loses the anchor the same way, because a blank diff line measures as indent 0. Both are
+# measured and filed as fleetd #639 — do not read this paragraph as a guarantee that a masked
+# key's value can never be printed.
 #
 # `redact` is always fed `diff -u` output, and every line of a unified diff starts with exactly
 # one of ' ', '+', '-' (the three body markers; '@'/'-'/'+' for the three header-line kinds too).