Merge branch 'nd/doc-index-format'

Update the index format documentation to mention the v4 format.

* nd/doc-index-format:
  update-index: list supported idx versions and their features
  read-cache.c: use INDEX_FORMAT_{LB,UB} in verify_hdr()
  index-format.txt: mention of v4 is missing in some places

diff --cc Documentation/git-update-index.txt
Simple merge

diff --cc Documentation/technical/index-format.txt
Simple merge

diff --cc read-cache.c
Simple merge