Various typo fixes #2 - only in comments, no code changes.

These files seem to be less-frequently touched, so I bundled them together in a pull-request to reduce the number of requests a bit.

Low priority fixes really, but I've been making some minor spelling and grammar corrections to the codebase, and will be submitting them as pull-requests a few files at a time to reduce the chance of any merge conflicts.

I will try to keep my branch as close to Marlin_v1 as possible, but if there are any conflicts just kick this back to me.