doc: vxworks: Mention chain-loading an x86 kernel via 'bootefi'

This updates the doc to mention chain-loading an x86 kernel via
'bootefi' command, along with several typos fix.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexander Graf <agraf@suse.de>