Can't figure out some sense from the git-commit-tree man page

Hi,

The git-commit-tree man page reads:
  A commit encapsulates:

  ·   all parent object ids

  ·   author name, email and date

  ·   committer name and email and the commit time.
  If not provided, "git-commit-tree" uses your name, hostname and domain to
  provide author and committer info. This can be overridden by either
  .git/config file, or using the following environment variables.
  (...)

The "If not provided" part doesn't make sense. If you're not setting the
author and committer in your config or environment, how are you supposed
to provide it ?

Naively, I tried sending a "git-cat-file commit" looking text to stdin,
but it just ended up in the changelog, which was actually no surprise.

Mike