btrfs-progs: docs: update bootloader section

Add reference to u-boot and primary super block offset.

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

Documentation/btrfs-man5.asciidoc

@@ -790,9 +790,15 @@ BOOTLOADER SUPPORT
 GRUB2 (https://www.gnu.org/software/grub) has the most advanced support of
 booting from BTRFS with respect to features.
 
+U-boot (https://www.denx.de/wiki/U-Boot/) has decent support for booting but
+not all BTRFS features are implemented, check the documentation.
+
 EXTLINUX (from the https://syslinux.org project) can boot but does not support
 all features. Please check the upstream documentation before you use it.
 
+The first 1MiB on each device is unused with the exception of primary
+superblock that is on the offset 64KiB and spans 4KiB.
+
 
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