Make explicit the straightforward nature of QDate::daysTo()

Pick-to: 6.11 6.8
Change-Id: Icc9955a034d8f8fa31cd26907129f13f306a7126
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 20a2b62)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

Author: ediosyncratic

src/corelib/time/qdatetime.cpp

@@ -1593,6 +1593,7 @@ QDate QDate::addYears(int nyears) const
 /*!
     Returns the number of days from this date to \a d.
 
+    This is equivalent to \c{d.toJulianDay() - toJulianDay()}.
     The result is negative if \a d is earlier than this date.
     Returns 0 if either date is invalid.