doc: Add documentation for swap() functions

The functions had been added without documentation.
This update adds the documentation.

Change-Id: Ib29e9356b26c248b5fcc9f13ecf77371fbae054a
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>

src/sql/models/qsqlrelationaltablemodel.cpp

@@ -99,6 +99,12 @@ typedef QSqlRelationalTableModelSql Sql;
     \sa tableName(), indexColumn(), displayColumn()
 */
 
+/*!
+  \fn void QSqlRelation::swap(QSqlRelation &other)
+
+  Swaps \c this with \a other.
+ */
+
 /*!
     \fn QString QSqlRelation::tableName() const
 

src/xml/sax/qxml.cpp

@@ -776,6 +776,11 @@ QXmlAttributes::~QXmlAttributes()
 {
 }
 
+/*!
+  \fn void QXmlAttributes::swap(QXmlAttributes &other)
+
+  Swaps \c this with \a other.
+ */
 
 /*!
     Looks up the index of an attribute by the qualified name \a qName.