gc docs: modernize the advice for manually running "gc"

gc docs: modernize the advice for manually running "gc"

The docs have been recommending that users need to run this manually,
but that hasn't been needed in practice for a long time except in
exceptional circumstances.

Let's instead have this reflect reality and say that most users don't
need to run this manually at all, while briefly describing the sorts
sort of cases where "gc" does need to be run manually.

Since we're recommending that users run this most of the and usually
don't need to tweak it, let's tone down the very prominent example of
the gc.auto=0 command. It's sufficient to point to the gc.auto
documentation below.

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