Document DateOnly.DayNumber translations (#374)

See npgsql/efcore.pg#3333

Author: roji

conceptual/EFCore.PG/mapping/translations.md

@@ -82,6 +82,9 @@ dateTime.Second                                                   | [date_part('
 dateTime.Year                                                     | [date_part('year', dateTime)::INT](https://www.postgresql.org/docs/current/functions-datetime.html)                                    |
 dateTime.ToUniversalTime                                          | [dateTime::timestamptz](https://www.postgresql.org/docs/current/datatype-datetime.html#id-1.5.7.13.18.7)                               | Added in 6.0
 dateTime.ToLocalTime                                              | [dateTime::timestamp](https://www.postgresql.org/docs/current/datatype-datetime.html#id-1.5.7.13.18.7)                                 | Added in 6.0
+dateOnly.DayNumber                                                | [dateOnly - DATE '0001-01-01'](https://www.postgresql.org/docs/current/functions-datetime.html)                                        | Added in 9.0
+DateOnly.FromDayNumber(x)                                         | [DATE '0001-01-01' + x](https://www.postgresql.org/docs/current/functions-datetime.html)                                               | Added in 9.0
+dateOnly1.DayNumber - dateOnly2.DayNumber                         | [dateOnly1 - dateOnly2](https://www.postgresql.org/docs/current/functions-datetime.html)                                               | Added in 9.0
 dateTimeOffset.DateTime                                           | [dateTimeOffset AT TIME ZONE 'UTC'](https://www.postgresql.org/docs/current/functions-datetime.html#FUNCTIONS-DATETIME-ZONECONVERT)    | Added in 6.0
 dateTimeOffset.UtcDateTime                                        | No PG operation (.NET-side conversion from DateTimeOffset to DateTime only)                                                            | Added in 6.0
 dateTimeOffset.LocalDateTime                                      | [dateTimeOffset::timestamp](https://www.postgresql.org/docs/current/datatype-datetime.html#id-1.5.7.13.18.7)                           | Added in 6.0

conceptual/EFCore.PG/release-notes/9.0.md

@@ -60,6 +60,7 @@ Thanks to [@ChrisJollyAU](https://github.com/ChrisJollyAU) and [@Timovzl](https:
 ## Other new features
 
 * Translate `array.Where(i => i != x)` to `array_remove(array, x)`
+* Translate `DateOnly.DayNumber`, `DateOnly.FromDayNumber()` and simplify `dateOnly1.DayNumber - dateOnly2.DayNumber` to `dateOnly1 - dateOnly2`.
 
 See the [9.0.0 milestone](https://github.com/npgsql/efcore.pg/milestone/61?closed=1) for the full list of Npgsql EF provider issues.