Update date picker documentation to include information about valid characters...

While working on gitlab-org/gitlab#39471 (closed), I noticed that the datepicker allows for alphanumeric characters. Those characters are invalid by default, and even though when entered into the date picker's input field they create an issue/milestone with the default date (today's), they should not be allowed.

This merge requests proposes the following: Update date picker documentation to include information about valid characters accepted into the input field.