It looks like the spell checker is a relatively recent addition. There are basic instructions on how to change the dictionary here.
Adding an unknown word to the dictionary requires alt + enter while the cursor is on the ‘misspelled’ word. This might take care of the most glaring omissions.
If it highlights just ‘doesn’, then it probably isn’t aware of English-style contractions (i.e., it doesn’t know that words can span across an apostrophe). Until that is fixed, I would recommend just adding ‘doesn’ as a separate word using the above method.