gc docs: note how --aggressive impacts --window & --depth

Since 07e7dbf0db (gc: default aggressive depth to 50, 2016-08-11) we
somewhat confusingly use the same depth under --aggressive as we do by
default.

As noted in that commit that makes sense, it was wrong to make more
depth the default for "aggressive", and thus save disk space at the
expense of runtime performance, which is usually the opposite of
someone who'd like "aggressive gc" wants.

But that's left us with a mostly-redundant configuration variable, so
let's clearly note in its documentation that it doesn't change the
default.

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

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