Clarify documentation of "git checkout <tree-ish> paths" syntax

Clarify documentation of "git checkout <tree-ish> paths" syntax

The SYNOPSIS section of the manual writes:

    git checkout [options] [<tree-ish>] [--] <paths>...

but the DESCRIPTION says that this form checks the paths out "from the
index, or from a named commit."  A later sentence refers to the same
argument as "<tree-ish> argument", but it is not clear that these two
sentences are talking about the same command line argument for first-time
readers.

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>