Implement parsing of the <Comment> tag

[ulikoehler]

Real-word example: The Stepperonline A6-EC servo drive, otherwise known as the Synmatic AS715N, download here: https://www.omc-stepperonline.com/de/a6-serie-100w-ethercat-ac-servomotor-kit-3000rpm-0-32nm-mit-bremse-17-bit-absolutwertgeber-ip67-a6-ec100l2b1-m17

Excerpt:

            <Device Physics="YY">
				<Type RevisionNo="#x00002EF8" ProductCode="#x00000715">A6N Servo Driver</Type>
				<Name LcId="1033">A6N_sAxis_V0.02</Name>
				<Comment LcId="1033">A6N Servo Driver</Comment>

It's not like the info is vitally important, but if we parse it, we can as well parse it properly.

Parsing it, at the moment, results in this error

$ cargo run --example basic STEPPERONLINE_A6_Servo_V0.02.xml
Error: Custom { kind: Other, error: "custom: unknown variant `Comment`, expected one of `Type`, `Name`, `RxPdo`, `TxPdo`, `Sm`, `Info`, `HideType`, `GroupType`, `URL`, `Profile`, `Eeprom`, `Fmmu`, `Image16x14`, `ImageFile16x14`, `ImageData16x14`, `Mailbox`, `Dc`, `Slots`" }

With the fix (excerpt, because it's really long):

EtherCatInfo {
   ...
        devices: [
            Device {
                physics: Some(
                    "YY",
                ),
                name: [
                    (
                        "A6N_sAxis_V0.02",
                        Some(
                            1033,
                        ),
                    ),
                ],
                desc: "A6N Servo Driver",
                comment: Some(
                    "A6N Servo Driver",
                ),