Bug 31156: Document borrowers-force-messaging-defaults.pl in cronjobs chapter

I added the script to the cronjob chapter even if it's not typically something someone would put in a crontab, but I just did it for a client, so you never know... I also did some spelling corrections in that same chapter. Feel free to edit the word list if you don't agree with some of the words there. I tried to add words that I don't think can be mistaken for typos, but I'm never sure for such specifics things.