From my research, Spell checking comments in the markups is not possible. It would be nice if it would be possible in the future.
When I perform a punch at a site, I am on my tablet and spelling is not a high priority with my fat fingers or those waiting for me to catch up in the next room. So when I get back to the office, I clean up the markups before issuing the punch list to the contractor.
I have created a custom remarks column, to add additional scope or clarifications for the contractor. I can see misspelled words but the spellcheck tool is not picking them up. Right now I have to go through each item to verify spelling and sometimes the list is long and I miss mistakes. By spell checking the comments, this will increase the quality of the reports with faster turnaround time.