fix cat-file usage message and documentation

cat-file with an object on the command line requires an
option to tell it what to output (type, size, pretty-print,
etc). However, the square brackets in the usage imply that
those options are not required. This patch switches them to
parentheses to indicate "required but grouped-OR" (curly
braces might also work, but this follows the convention used
already by "git stash").

While we're at it, let's change the <sha1> specifier in the
usage to <object>. That's what the documentation uses, and
it does actually use the regular object lookup.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-cat-file.txt

@@ -9,8 +9,8 @@ git-cat-file - Provide content or type and size information for repository objec
 SYNOPSIS
 --------
 [verse]
-'git cat-file' [-t | -s | -e | -p | <type>] <object>
-'git cat-file' [--batch | --batch-check] < <list-of-objects>
+'git cat-file' (-t | -s | -e | -p | <type>) <object>
+'git cat-file' (--batch | --batch-check) < <list-of-objects>
 
 DESCRIPTION
 -----------

builtin-cat-file.c

@@ -201,8 +201,8 @@ static int batch_objects(int print_contents)
 }
 
 static const char * const cat_file_usage[] = {
-	"git cat-file [-t|-s|-e|-p|<type>] <sha1>",
-	"git cat-file [--batch|--batch-check] < <list_of_sha1s>",
+	"git cat-file (-t|-s|-e|-p|<type>) <object>",
+	"git cat-file (--batch|--batch-check) < <list_of_objects>",
 	NULL
 };