org manual corrections (mostly English grammar)

Trying to make documentation more readable, tried to correct grammar and English expressions in general. So it's easier to read for the regular English speaker. Please also check line 1690 (starting on TODO) since I wasn't able to fix that sentence that seems to contradict itself.

I also changed some sentences fully guessing what it was meant, but I may not have guessed right sometimes, please check. This manual is far from perfect, but proper readable English should make it at least one step closer.