Mention QRect's int min/max constraints in detailed description.

Task-number: QTBUG-25732

Change-Id: If330768c3075568f09593ed17f26389d3dec3335
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
QDnsLookup: test MX and SRV records that can change order

The DNS protocol changes the order of the RRs in each reply it sends,
in an effort to balance the load in servers. For most tests, to ensure
that we get always the same result, we simply sort it back.

For MX and SRV, we can't sort because we also need to test that
QDnsLookup sorted correctly according to priority. So instead allow
that test to have multiple alternatives.

Change-Id: I5c119f907b31789de5c9cf2471cc82ecd140d06f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
QDnsLookup: test multiple TXT answers too

The TXT record is particular because each RR can contain multiple text
strings. So we need to join each RR's texts too.

To make it easy, I've made everything be QStrings.

Change-Id: Ia0506544b913585e7be860c81077cff8e0dab547
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>