config doc: unify the description of fsck.* and receive.fsck.*

config doc: unify the description of fsck.* and receive.fsck.*

The documentation for the fsck.<msg-id> and receive.fsck.<msg-id>
variables was mostly duplicated in two places, with fsck.<msg-id>
making no mention of the corresponding receive.fsck.<msg-id>, and the
same for fsck.skipList.

I spent quite a lot of time today wondering why setting the
fsck.<msg-id> variant wasn't working to clone a legacy repository (not
that that would have worked anyway, but a subsequent patch implements
fetch.fsck.<msg-id>).

Rectify this situation by describing the feature in general terms
under the fsck.* documentation, and make the receive.fsck.*
documentation refer to those variables instead.

This documentation was initially added in 2becf00ff7 ("fsck: support
demoting errors to warnings", 2015-06-22) and 4b55b9b479 ("fsck:
document the new receive.fsck.<msg-id> options", 2015-06-22).

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