docs: acpi_hest_ghes: fix documentation for CPER size

docs: acpi_hest_ghes: fix documentation for CPER size

While the spec defines a CPER size of 4KiB for each record,
currently it is set to 1KiB. Fix the documentation and add
a pointer to the macro name there, as this may help to keep
it updated.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <f7e94433bec19a9d6b23ecccc24b5fe3a6f7f52b.1736945236.git.mchehab+huawei@kernel.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>