docs: filesystems: add info about efivars content

When an EFI variable is reading from:

/sys/firmware/efi/efivars

(for example using "hexdump"), the first 4 bytes of the
output are not the real EFI variable data, but the variable
attributes (in little-endian format).

Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Link: https://lore.kernel.org/r/20200519084128.12756-1-f.suligoi@asem.it


Signed-off-by: Jonathan Corbet <corbet@lwn.net>