Align directive-order wording in the pointer target

Address CoPilot review on #226: the _SUPPORTED_FIXTURES comment points
readers at _apply_metadata_directives for the document-order behavior,
but that function's own comment still said "IN KEY ORDER" / "YAML key
order" -- the sorted-by-key ambiguity this branch removes at the call
site. Reword it to "document order" so both spots agree.

Comment-only; no behavior change.

Author: chris-colinsky

tests/conformance/test_observability.py

@@ -1590,8 +1590,9 @@ def _apply_metadata_directives(
 ) -> tuple[dict[str, Any], bool]:
     """Run a node's (or per-attempt's) in-node metadata directives, returning the
     resulting state update and whether the node should then raise."""
-    # Directives run IN KEY ORDER -- 043 augments then captures, 045 attempt 1
-    # captures then augments, and the YAML key order encodes that. The capture
+    # Directives run IN DOCUMENT ORDER -- 043 augments then captures, 045
+    # attempt 1 captures then augments, and the YAML document order encodes
+    # that. The capture
     # records the read's type so the immutability invariant can verify it was a
     # MappingProxyType. ``raises`` is terminal (a real node body that raises runs
     # nothing after), so stop at it rather than processing later directives.