add better comment for NetdSELinuxTest::CheckProperMTULabels test

Test: N/A, comments only.
Bug: 185962988
Bug: 190048216
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Original-Change: https://android-review.googlesource.com/1730981
Merged-In: Ib23710505386960d5d436e1afc89d7c1c33e9ee2
Change-Id: Ib23710505386960d5d436e1afc89d7c1c33e9ee2