device paths: improve documentation to construct them

[phip1611]

I currently try to extend a device path pointing to a FAT file system with path components, without success. It also seems we do not have documentation for that.

If this is possible, we should document it and add an example, perhaps even a test.

[phip1611]

Pseudo code:

let sfp_protocol = unsafe {
            boot::open_protocol::<SimpleFileSystem>(
                OpenProtocolParams {
                    handle: *handle,
                    agent: boot::image_handle(),
                    controller: None,
                },
                OpenProtocolAttributes::GetProtocol,
            )?
        };

        // Device path of the handle
        let dp = unsafe {
            boot::open_protocol::<DevicePath>(
                OpenProtocolParams {
                    handle: *handle,
                    agent: boot::image_handle(),
                    controller: None,
                },
                OpenProtocolAttributes::GetProtocol,
            )?
        };
        let p = FilePath {
            path_name: PATH
        };
        let node: &DevicePathNode = DevicePathNode::try_from(p)?;
        dp.append_node(node)?;