QRE: discourage users from assuming that QRE stores the subject

When matching over a QString we store a (shallow) copy of it.
That has always been an implementation detail and people should've
never relied on it, but Hyrum's law, we don't want to actually
exploit this by NOT taking the copy.

Converesely, matching over QStringView already requires that
the string data is kept alive as long as QRE(Match) objects
are alive.

Add a doc note to give us the freedom to do the change in Qt 7.

[ChangeLog][QtCore][QRegularExpression] QRegularExpression takes
a shallow copy of a QString subject when matching over it. This
means that users can destroy or modify the string data while
still having match results over it. This behavior is deprecated;
in a future version of Qt, QRegularExpression will no longer
take a copy. Note that behavior has always been undocumented and
users were never supposed to modify a subject string while
match objects were alive on it. In practice, it's very unlikely
that your code is relying on the existing behavior.

Change-Id: Ibc5f900c09a007139fb12fc4d7f11e4a8f31bf38
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qregularexpression.cpp

@@ -1571,6 +1571,11 @@ qsizetype QRegularExpression::patternErrorOffset() const
     The returned QRegularExpressionMatch object contains the results of the
     match.
 
+    \note The data referenced by \a subject should remain valid as long
+    as there are QRegularExpressionMatch objects using it. At the moment
+    Qt makes a (shallow) copy of the data, but this behavior may change
+    in a future version of Qt.
+
     \sa QRegularExpressionMatch, {normal matching}
 */
 QRegularExpressionMatch QRegularExpression::match(const QString &subject,
@@ -1628,6 +1633,11 @@ QRegularExpressionMatch QRegularExpression::match(QStringView subjectView,
     The returned QRegularExpressionMatchIterator is positioned before the
     first match result (if any).
 
+    \note The data referenced by \a subject should remain valid as long
+    as there are QRegularExpressionMatch objects using it. At the moment
+    Qt makes a (shallow) copy of the data, but this behavior may change
+    in a future version of Qt.
+
     \sa QRegularExpressionMatchIterator, {global matching}
 */
 QRegularExpressionMatchIterator QRegularExpression::globalMatch(const QString &subject,