docs: Document workaround for grub-mkrescue with older MacBooks

Add a description of the workaround for firmware of older MacBooks
which stalls with a grub-mkrescue ISO image for x86_64-efi target
on an USB stick.

Signed-off-by: Thomas Schmitt <scdbackup@gmx.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Author: scdbackup

docs/grub.texi

@@ -6131,6 +6131,7 @@ Required files are:
 
 @menu
 * GRUB only offers a rescue shell::
+* Firmware stalls instead of booting GRUB::
 @end menu
 
 
@@ -6201,6 +6202,17 @@ support has not yet been added to GRUB.
 @end itemize
 
 
+@node Firmware stalls instead of booting GRUB
+@section Firmware stalls instead of booting GRUB
+
+The EFI implementation of some older MacBook laptops stalls when it gets
+presented a grub-mkrescue ISO image for x86_64-efi target on an USB stick.
+Affected are models of year 2010 or earlier. Workaround is to zeroize the
+bytes 446 to 461 of the EFI partition, where mformat has put a partition table
+entry which claims partition start at block 0. This change will not hamper
+bootability on other machines.
+
+
 @node Invoking grub-install
 @chapter Invoking grub-install