Merge branch 'path-lookup' into docs-next

From Fox Chen:

  The Path lookup is a very complex subject in VFS. The path-lookup
  document provides a very detailed guidance to help people understand how
  path lookup works in the kernel. This document was originally written
  based on three lwn articles five years ago. As times goes by, some of the
  content is outdated. This patchset is intended to update the document to
  make it more relevant to current codebase.