[bbq2]: Confusing docs for ReadGrantError

[gsson]

Hi!

Having a play with bbq2 and noticed the following

/// Errors associated with obtaining a read grant
#[derive(PartialEq, Debug)]
pub enum ReadGrantError {
    /// Unable to create write grant due to not enough room in the buffer
    Empty,
    /// Unable to create write grant due to existing write grant
    GrantInProgress,
    /// We observed a frame header that did not make sense. This should only
    /// occur if a stream producer was used on one end and a frame consumer was
    /// used on the other end. Don't do that.
    ///
    /// If you see this error and you are NOT doing that, please report it, as it
    /// is a bug.
    InconsistentFrameHeader,
}

I would have expected the first two to read something like this:

/// Errors associated with obtaining a read grant
#[derive(PartialEq, Debug)]
pub enum ReadGrantError {
    /// Unable to create read grant due to insufficient data in the buffer
    Empty,
    /// Unable to create read grant due to existing read grant
    GrantInProgress,
   ...
}

I have some other nits/issues I might want to file, but I figured I'd start with something easy :)