Provide commentary on _why_ the sync function should exist.

[MicahZoltu]

https://github.com/Uniswap/uniswap-v2-core/blob/d2ddbd95e8500cfd3669ab81b64f48956521ea6b/contracts/UniswapV2.sol#L158-L161

I believe that this function exists to deal with some obscure edge cases (rounding perhaps)/attacks as it allows one to "reset" the exchange if it somehow ends up short on capital (e.g., the token contract burns Uniswap balances either on purpose or on accident). However, these situations are incredibly obscure and while I normally don't advocate for a lot of comments on code (the code should be easy to read without comments), in cases like this where the why is not obvious I believe there is significant value.

[haydenadams]

I believe that this function exists to deal with some obscure edge cases (rounding perhaps)/attacks as it allows one to "reset" the exchange if it somehow ends up short on capital

Correct!

[NoahZinsmeister]

closed by #45