Various doc & formatting fixes in API classes

I made some changes to ensure the JavaDocs were mainly compliant with the contribution guidelines.

**Readability:**  
- When adjusting wrapping of lines, I made sure to leave (at least) 2 words on the next line

**Consistency:**  
- Added some missing 'periods' (full stops for those among us who hail from the UK)
- Wrapped some lines if they exceeded 80 characters
- Correctly spaced some doc tags (`@return`, `@param`, etc.)
- Also did some spacing in the code where there should be some

**Exceptions:**  
- I had some trouble with the `Order.java` class. The table formatting admittedly threw me off a tad.

Also, in `Plugin.java`, I'd like to ask whether lines 62-83 are correct. I tried to space it out a little but it looks a bit weird.

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If you want someone to maintain the documentation in the way that I did here, I'd be glad to do it. [I did a major reformat of nearly all the classes in the Bukkit API JavaDocs](https://github.com/Bukkit/Bukkit-JavaDoc/commits?author=aerouk), so I feel I've got the gist of how a role like this works.

Cheers!