QuerySet .distinct([*fields]) documentation improvements

[Jani Tiainen]

QuerySet does have .distinct([*fields]) method has optional feature to provide fields that are used on distinct. This feature is supported only by PostgreSQL.

In documentation there is a small note about fields being supported in PostgreSQL only. Almost all examples though uses fields provided which works only in PostgreSQL. It gives impression if reader does just a quick look of examples that passing parameters is common operation which works everywhere.

Examples should use common format that works for everybody and with a note add database spesifics.

[Anssi Kääriäinen]

I think there is room for some improvement here. Maybe the whole thing should be split to .distinct() and .distinct(*fields)?

[Tim Graham <timograham@…>]

In 577b0f91894469872974ad609b4fabca949cdc1b:

Fixed #20561 -- Emphasized that QuerySet.distinct([*fields]) is only supported by Postgres.

Thanks jtiai for the suggestion.

[Tim Graham <timograham@…>]

In 430aae1b0db9fbcc15415b7bd9a14df1d88359cf:

[1.6.x] Fixed #20561 -- Emphasized that QuerySet.distinct([*fields]) is only supported by Postgres.

Thanks jtiai for the suggestion.

Backport of 577b0f9189 from master.