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).
Any reason for changing comments *within* functions? Does doxygen ever show them? `//!` is meant for descriptions of variables and properties AFAIK.