fix cat-file usage message and documentation

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>