Addrman doxygenification

First look at changing comments to be doxygen compatible. Started with addrman.cpp/.h because they don't see many changes, and have a decent amount of comments/doc thoughout. This pull includes minor typo fixes and updating the license in both files; as well as a commit that adds doc/doxygen to .gitignore

@laanwj What would you prefer be done with the larger doc blocks, i.e [here](https://github.com/bitcoin/bitcoin/blob/e7a00a37746906048f649d622a42f9b347110ab6/src/addrman.h#L101).
IMO it's better to just touch the API documenting comments for now, not those in functions.
(so those documenting the behaviour of functions, methods, classes, properties, etc...). I don't like to change every single comment in the program and require a specific comment style in functions.