Add notice for desktop device type

[Simonoob]

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UAParser doesn't report desktop as a device type because the info is not accessible via the UA string.

While this makes sense, at first glance one can assume that the library will report such type, since other "similar" devices are returned (mobile, tablet etc.).

I personally found about this only when running into issues with desktop devices not being detected, and there have been multiple issues opened for similar reasons.

A small note in the docs can clarify this behaviour and avoid future headaches