gc docs: stop noting "repack" flags

gc docs: stop noting "repack" flags

Remove the mention of specific flags from the "gc" documentation, and
leave it at describing what we'll do instead. As seen in builtin/gc.c
we'll use various repack flags depending on what we detect we need to
do, so this isn't always accurate.

More importantly, a subsequent change is about to remove all this
documentation and replace it with an include of the gc.* docs in
git-config(1). By first changing this it's easier to reason about that
subsequent change.

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