[LayoutNG] Update BFC (block) offset resolving documentation.

This was pretty mechanical, but everything I think still makes sense.
Effectively everything that references "resolving a BFC offset" now
turns into "resolving a BFC block offset".

There should be no behaviour change.

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I1215520ab21eae6fb45b4ab53be08a0fdde232ac
Bug: 635619
Reviewed-on: https://chromium-review.googlesource.com/1155266
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579816}
6 files changed