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>

docs/specs/acpi_hest_ghes.rst

@@ -67,8 +67,10 @@ Design Details
 (3) The address registers table contains N Error Block Address entries
     and N Read Ack Register entries. The size for each entry is 8-byte.
     The Error Status Data Block table contains N Error Status Data Block
-    entries. The size for each entry is 4096(0x1000) bytes. The total size
-    for the "etc/hardware_errors" fw_cfg blob is (N * 8 * 2 + N * 4096) bytes.
+    entries. The size for each entry is defined at the source code as
+    ACPI_GHES_MAX_RAW_DATA_LENGTH (currently 1024 bytes). The total size
+    for the "etc/hardware_errors" fw_cfg blob is
+    (N * 8 * 2 + N * ACPI_GHES_MAX_RAW_DATA_LENGTH) bytes.
     N is the number of the kinds of hardware error sources.
 
 (4) QEMU generates the ACPI linker/loader script for the firmware. The