Add Dragon bug to date input guidance

[hannalaakso]

alphagov/govuk-frontend#1101 is not a problem with our implementation but with Dragon and it's been reported to them. We should try to reproduce this to confirm the Dragon versions and browsers affected and then document the problem. I spoke to Amy and we agreed that it would be good to put this under "Known issues and gaps" in the date input guidance, similar to what we do with the character count.

[36degrees]

I think once we've tackled alphagov/govuk-frontend#1449 we can close this without updating the guidance as the Dragon bug will no longer affect the date input component.

[36degrees]

As documented in alphagov/govuk-frontend#1101 this has now been fixed, so we do not need to document it.