doc: correct spellcheck and misspellings

Documentation spellcheck reported a handful of misspelled words. Many
were false positives, and were added to the allowlist. Any spelling
mistakes have been corrected.

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>
Change-Id: I9ce90ff0ea6af628a7e685e64034acab86649d93