Cleanup and update comments SA mark disabling comments

This commit cleans up and upates comments with regard to changes in
aosp/721999, clarifying the restrictions and potential pitfalls we would
see with regards to IPsec tunnel mode without updatable SAs.

Bug: 111854872
Test: Compiles, comment-only change
Change-Id: I07b0063987463c1a3cf42e112839a31739947c80