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>

Documentation/git-checkout.txt

@@ -27,7 +27,7 @@ the first namespace level.
 
 When <paths> are given, this command does *not* switch
 branches.  It updates the named paths in the working tree from
-the index file, or from a named commit.  In
+the index file, or from a named <tree-ish> (most often a commit).  In
 this case, the `-b` options is meaningless and giving
 either of them results in an error.  <tree-ish> argument can be
 used to specify a specific tree-ish (i.e. commit, tag or tree)