Documentation for Internal Functions

Documentation is nice to define inputs/outputs and what a function does. Extending this out to internal functions would be nice as well, especially when coming back to revise the function. However, I don't think it's necessary to outline a full docstr for these internal functions.

Any thoughts on this? Suggestions for a format? Maybe something like,

function _foobar(x, y; z)
    """
    Explanation of what foobar does, maybe explain why it does things in a certain way?
    Have docstr inside the function for quick glance value that a function is internal?
    """
end

The issue names no file or test; start with the linked BlueStyle documentation and the internal-function example in the issue. Decide whether a reduced documentation format is appropriate, then document the agreed convention in the style guide so the expected format is clear.