ICU-21282 Fixup date patterns and date field symbols display

ICU-21282 Fixup date patterns and date field symbols display

No content changes.  This make the display of date patterns, date field symbols, and skeletons consistent.
- Date field symbols are in a code block (e.g. `y` or `yyyy`)
- Date patterns are quoted in a code block (e.g. `"y"` or `"MMMM d 'at' h:mm a"`)
- Skeletons are the same as date patterns (e.g. `"MMMMdjmm"`)
- Strings that might be the result of a date pattern are quoted (e.g. "April 2 at 5:00 PM")
Thus `"y"` denotes the date pattern containing the date field symbol `y`