Make NodeInsertMultipleEntry private and clarify some comments in
IntermTraverse.h.

BUG=angleproject:2100
TEST=angle_unittests, angle_end2end_tests

Change-Id: Iae60a46714c8b5cb9ad1e9d70aa6776f9deaf3d5
Reviewed-on: https://chromium-review.googlesource.com/715718Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>