main loop: add missing documentation links to GS/IO macros

main loop: add missing documentation links to GS/IO macros

If we go directly to GLOBAL_STATE_CODE, IO_CODE or IO_OR_GS_CODE
definition, we just find that they "mark and check that the function
is part of the {category} API".
However, ther is no definition on what {category} API is, they are
in include/block/block-*.h
Therefore, add a comment that refers to such documentation.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20220609122206.1016936-1-eesposit@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>