QString: improve lastIndexOf(QRegularExpression) docs

Due to how regex matching works, lastIndexOf has to be implemented
by matching from the string's beginning until the from position
is reached. This might not be obvious for users, so document that.

Task-number: QTBUG-89050
Pick-to: 5.15 6.0
Change-Id: I4b69ea753e7d417d980031926f1e01d77e58720d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qstring.cpp

@@ -4261,6 +4261,10 @@ qsizetype QString::indexOf(const QRegularExpression &re, qsizetype from, QRegula
     Example:
 
     \snippet qstring/main.cpp 94
+
+    \note Due to how the regular expression matching algorithm works,
+    this function will actually match repeatedly from the beginning of
+    the string until the position \a from is reached.
 */
 qsizetype QString::lastIndexOf(const QRegularExpression &re, qsizetype from, QRegularExpressionMatch *rmatch) const
 {