cat-file docs: fix SYNOPSIS and "-h" output

cat-file docs: fix SYNOPSIS and "-h" output

There were various inaccuracies in the previous SYNOPSIS output,
e.g. "--path" is not something that can optionally go with any options
except --textconv or --filters, as the output implied.

The opening line of the DESCRIPTION section is also "In its first
form[...]", which refers to "git cat-file <type> <object>", but the
SYNOPSIS section wasn't showing that as the first form!

That part of the documentation made sense in
d83a42f34a6 (Documentation: minor grammatical fixes in
git-cat-file.txt, 2009-03-22) when it was introduced, but since then
various options that were added have made that intro make no sense in
the context it was in. Now the two will match again.

The usage output here is not properly aligned on "master" currently,
but will be with my in-flight 4631cfc20bd (parse-options: properly
align continued usage output, 2021-09-21), so let's indent things
correctly in the C code in anticipation of that.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>