Spelling fixes for the docs

It's complicated. Most of the time you use an before a vowel or a h if the h is followed by a vowel but there are some exceptions. If the word starts with 'un' it is usually preceded by 'a' as it is too hard to say 'an university'. I'm not sure of the exact rules but think of it like 'readability counts', if it would be hard to say 'an X' then it might be 'a' instead of 'an'