Fix spelling, grammar and typo mistakes in the documentation

I'm a new user. I was reading the documentation to familiarize myself with the plug-in and  came across some typos and spelling mistakes. So I decided to go through the documentation, the best I could, and correct any issues which stood out to me. I purposefully was not too aggressive. If I didn't understand something I didn't change it, but you can look over my changes to verify I did not unintentionally change the meaning of something.