Add XML documentation for DatePicker DateTime properties

[Copilot]

The .NET-friendly DatePicker date properties lacked IntelliSense guidance about month indexing and Android millisecond timestamps.

• DateTime

  • Document the .NET 1-based to Android 0-based month conversion.
  • Clarify that only year, month, and day are used.

• MinDateTime and MaxDateTime

  • Document conversion from Android milliseconds relative to January 1, 1970.

• API references

  • Link each property to the relevant official Android DatePicker documentation.

• Fixes [fix-finder] Add XML documentation to DatePicker DateTime properties #12305

[Copilot]

Pull request overview

This PR adds missing XML documentation to the handwritten, .NET-friendly Android.Widget.DatePicker date properties in Mono.Android.dll, improving IntelliSense guidance around month indexing and Android millisecond-based date values (fixes #12305).

Changes:

• Added <summary> / <remarks> for DatePicker.DateTime explaining .NET 1-based vs Android 0-based month conversion and that only year/month/day are used when setting.
• Added <summary> / <value> for MinDateTime and MaxDateTime describing conversion from Android’s millisecond-based date values.
• Added <seealso> links to the relevant official Android DatePicker reference pages.

Suppressed comments (1)

src/Mono.Android/Android.Widget/DatePicker.cs:25

• 🤖 💡 Documentation — Same as getMinDate(): Android’s DatePicker.getMaxDate() is defined in terms of milliseconds since Jan 1, 1970 00:00:00 in the device’s default time zone. The <value> text would be clearer if it included the time zone context.

		/// <summary>Gets the maximum selectable date supported by this picker.</summary>
		/// <value>The maximum date converted from Android's millisecond value relative to January 1, 1970.</value>
		/// <seealso href="https://developer.android.com/reference/android/widget/DatePicker#getMaxDate()">Android documentation for <c>android.widget.DatePicker.getMaxDate</c></seealso>