docs(diff): record verified cached diff-scan param behaviour

Probed the live API against an existing diff scan to confirm what the polling path
can and cannot ask for:

- omit_license_details is ignored when cached=true, as the existing comment said.
  License fields remain in the response.
- omit_unchanged IS honored and removes unchanged artifacts entirely, measured at
  ~1.1 KB per artifact (225,542 B -> 78,003 B when dropping 135 of 192 artifacts).

Record why the CLI still does not send omit_unchanged: unchanged artifacts feed
diff.unchanged_alerts, which create_security_comment_gitlab and the FOSSA compat
issue list read unconditionally, not only under --strict-blocking. Omitting them
would silently shrink those outputs, so this needs proper gating in its own change
rather than a param tweak here.

Ref: CE-379

Author: lelia

socketsecurity/core/__init__.py

@@ -1605,6 +1605,16 @@ def get_diff_scan_artifacts(
         # payload ever gets a response truncated on a huge dependency tree,
         # response.json() fails and the caller falls back to the legacy
         # streaming comparison, which still requests the lean payload.
+        #
+        # Verified against the live API: passing omit_license_details alongside
+        # cached=true leaves the license fields in the response, but omit_unchanged
+        # IS honored and drops the unchanged artifacts entirely (~1.1 KB each). Not
+        # sent here because unchanged artifacts are not only used by
+        # --strict-blocking: create_security_comment_gitlab and the FOSSA compat
+        # issue list both read diff.unchanged_alerts unconditionally, so omitting
+        # them would silently shrink those outputs. Gating it correctly across all
+        # three consumers is worth doing - on a tree with ~10k unchanged artifacts
+        # it is over 10 MB of response - but it needs its own change.
         poll_params = {"cached": "true"}
         poll_start = time.monotonic()
         deadline = poll_start + DIFF_SCAN_POLL_TIMEOUT_SECONDS