docs: resolve sphinx spelling check warnings

- Fix some spelling errors in docs.
- Update words list for sphinx spelling check.

Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Lijian Zhang <lijian.zhang@arm.com>
Change-Id: I18f9aa3be6c7b60f2cbefc17f9b4248cd9f76d6c