mktree --missing: updated usage message and man page

Update usage message in builtin-mktree.c to include '--missing'.  Do the
same to man page and clarify that the input does not have to be sorted.

Signed-off-by: Josh Micich <josh.micich@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-mktree.txt

@@ -8,19 +8,26 @@ git-mktree - Build a tree-object from ls-tree formatted text
 
 SYNOPSIS
 --------
-'git mktree' [-z]
+'git mktree' [-z] [--missing]
 
 DESCRIPTION
 -----------
-Reads standard input in non-recursive `ls-tree` output format,
-and creates a tree object.  The object name of the tree object
+Reads standard input in non-recursive `ls-tree` output format, and creates
+a tree object.  The order of the tree entries is normalised by mktree so
+pre-sorting the input is not required.  The object name of the tree object
 built is written to the standard output.
 
 OPTIONS
 -------
 -z::
 	Read the NUL-terminated `ls-tree -z` output instead.
 
+--missing::
+	Allow missing objects.  The default behaviour (without this option)
+	is to verify that each tree entry's sha1 identifies an existing
+	object.  This option has no effect on the treatment of gitlink entries
+	(aka "submodules") which are always allowed to be missing.
+
 Author
 ------
 Written by Junio C Hamano <gitster@pobox.com>

builtin-mktree.c

@@ -63,7 +63,7 @@ static void write_tree(unsigned char *sha1)
 }
 
 static const char *mktree_usage[] = {
-	"git mktree [-z]",
+	"git mktree [-z] [--missing]",
 	NULL
 };