gc docs: re-flow the "gc.*" section in "config"

gc docs: re-flow the "gc.*" section in "config"

Re-flow the "gc.*" section in "config". A previous commit moved this
over from the "gc" docs, but tried to keep as many of the lines
identical to benefit from diff's move detection.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>