fix diff-tree --stdin documentation

fix diff-tree --stdin documentation

Long time ago, the feature of "diff-tree --stdin" to take a commit and its
parents on one line was broken, and did not support the common:

    git rev-list --parents $commits... -- $paths... |
    git diff-tree --stdin -v -p

usage pattern by Porcelains properly.  For diff-tree to talk sensibly
about commits, it needs to see commits, not just trees; the code was fixed
to take list of commits on the standard input in 1.2.0.

However we left the documentation stale for a long time, until Karl
Hasselström finally noticed it very recently.

Signed-off-by: Junio C Hamano <gitster@pobox.com>