docs: add JSDoc for attributes [skip ci] (#43)

src/vaadin-date-time-picker.html

@@ -196,6 +196,7 @@
 
             /**
              * The error message to display when the input is invalid.
+             * @attr {string} error-message
              */
             errorMessage: String,
 
@@ -265,20 +266,23 @@
 
             /**
              * A placeholder string for the date field.
+             * @attr {string} date-placeholder
              */
             datePlaceholder: {
               type: String
             },
 
             /**
              * A placeholder string for the time field.
+             * @attr {string} time-placeholder
              */
             timePlaceholder: {
               type: String
             },
 
             /**
              * String used for the helper text.
+             * @attr {string} helper-text
              */
             helperText: {
               type: String,
@@ -309,13 +313,15 @@
              * Date which should be visible in the date picker overlay when there is no value selected.
              *
              * The same date formats as for the `value` property are supported but without the time part.
+             * @attr {string} initial-position
              */
             initialPosition: String,
 
             /**
              * Set true to display ISO-8601 week numbers in the calendar. Notice that
              * displaying week numbers is only supported when `i18n.firstDayOfWeek`
              * is 1 (Monday).
+             * @attr {boolean} show-week-numbers
              */
             showWeekNumbers: {
               type: Boolean
@@ -332,6 +338,7 @@
 
             /**
              * Set to true to prevent the overlays from opening automatically.
+             * @attr {boolean} auto-open-disabled
              */
             autoOpenDisabled: Boolean,