doc: Add note to Secure Firmware Update for expected errors

In the Secure Firmware Update, when the firmware update starts,
RSE writes the image into the flash correctly. Then both
RSE and U-Boot restart. U-Boot will require the RSE to write
the image again. Since the image version has been updated, RSE
will report a version error message.

Add a note in the documentation to tell the user that this
is an expected behavior.

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>