Document uses of BASIC_SELL

[duncancmt]

          Ah I see, so fees, deposit/withdraw wrapped native responsibilities got rolled up into the general `BASIC_SELL`. Now that I am aware of this the code makes more sense, in my mind I was still thinking historically where it just supported generic dexes, and WETH functionality would be added as a separate action.

We should document this intention as the current docs only mention that it sells to a generic pool that follow the approval, transferFrom interaction pattern.

I don't necessarily mind a fanciful BASIC_SELL, to save on codesize. It is less clear to an observer though, especially when used as a fee transfer mechanism rather than a sell.

Originally posted by @dekz in #45 (comment)