Doc: fix warnings for QSetIterator see-alsos

Amends 0306247f5a, which removed some APIs
from QSetIterator as they couldn't be implemented without operator--.
That resulted in qdoc warnings, which this patch removes by splitting
the QSetIterator documentation into a separate block that quotes the
previously shared documentation text, but uses a reduced set of see-also
links.

Change-Id: I2aac59b927a36216a718aa8e5d092ea4d9f6c15b
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>

src/corelib/doc/snippets/code/doc_src_qiterator.cpp

@@ -25,15 +25,6 @@ while (i.hasNext())
     float f = i.next();
 //! [6]
 
-
-//! [7]
-QSetIterator<QString> i(set);
-i.toBack();
-while (i.hasPrevious())
-    QString s = i.previous();
-//! [7]
-
-
 //! [8]
 QList<float> list;
 ...

src/corelib/tools/qiterator.qdoc

@@ -213,12 +213,8 @@
 
     \image javaiterators1.png
 
-    Here's how to iterate over the elements in reverse order:
-
-    \snippet code/doc_src_qiterator.cpp 7
-
     If you want to find all occurrences of a particular value, use
-    findNext() or findPrevious() in a loop.
+    findNext() in a loop.
 
     Multiple iterators can be used on the same set. If the set
     is modified while a QSetIterator is active, the QSetIterator
@@ -389,15 +385,21 @@
 */
 
 /*! \fn template <class T> void QListIterator<T>::toBack()
-    \fn template <class T> void QSetIterator<T>::toBack()
     \fn template <class T> void QMutableListIterator<T>::toBack()
 
+//! [toBack]
     Moves the iterator to the back of the container (after the last
     item).
+//! [toBack]
 
     \sa toFront(), previous()
 */
 
+/*! \fn template <class T> void QSetIterator<T>::toBack()
+    \include qiterator.qdoc toBack
+    \sa toFront()
+*/
+
 /*! \fn template <class T> void QMutableSetIterator<T>::toBack()
 
     Moves the iterator to the back of the container (after the last
@@ -407,16 +409,22 @@
 */
 
 /*! \fn template <class T> bool QListIterator<T>::hasNext() const
-    \fn template <class T> bool QSetIterator<T>::hasNext() const
     \fn template <class T> bool QMutableListIterator<T>::hasNext() const
 
+//! [hasNext]
     Returns \c true if there is at least one item ahead of the iterator,
     i.e. the iterator is \e not at the back of the container;
     otherwise returns \c false.
+//! [hasNext]
 
     \sa hasPrevious(), next()
 */
 
+/*! \fn template <class T> bool QSetIterator<T>::hasNext() const
+    \include qiterator.qdoc hasNext
+    \sa next()
+*/
+
 /*! \fn template <class T> bool QMutableSetIterator<T>::hasNext() const
 
     Returns \c true if there is at least one item ahead of the iterator,
@@ -427,16 +435,23 @@
 */
 
 /*! \fn template <class T> const T &QListIterator<T>::next()
-    \fn template <class T> const T &QSetIterator<T>::next()
 
+//! [next]
     Returns the next item and advances the iterator by one position.
 
     Calling this function on an iterator located at the back of the
     container leads to undefined results.
+//! [next]
 
     \sa hasNext(), peekNext(), previous()
 */
 
+/*!
+    \fn template <class T> const T &QSetIterator<T>::next()
+    \include qiterator.qdoc next
+    \sa hasNext(), peekNext()
+*/
+
 /* \fn template <class T> const T &QMutableSetIterator<T>::next()
     Returns the next item and advances the iterator by one position.
 
@@ -468,16 +483,23 @@
 */
 
 /*! \fn template <class T> const T &QListIterator<T>::peekNext() const
-    \fn template <class T> const T &QSetIterator<T>::peekNext() const
 
+//! [peekNext]
     Returns the next item without moving the iterator.
 
     Calling this function on an iterator located at the back of the
     container leads to undefined results.
+//! [peekNext]
 
     \sa hasNext(), next(), peekPrevious()
 */
 
+/*!
+    \fn template <class T> const T &QSetIterator<T>::peekNext() const
+    \include qiterator.qdoc peekNext
+    \sa hasNext(), next()
+*/
+
 /*!
     \fn template <class T> const T &QMutableSetIterator<T>::peekNext() const
 
@@ -563,19 +585,24 @@
 */
 
 /*! \fn template <class T> bool QListIterator<T>::findNext(const T &value)
-    \fn template <class T> bool QSetIterator<T>::findNext(const T &value)
     \fn template <class T> bool QMutableListIterator<T>::findNext(const T &value)
 
+//! [findNext]
     Searches for \a value starting from the current iterator position
     forward. Returns \c true if \a value is found; otherwise returns \c false.
 
     After the call, if \a value was found, the iterator is positioned
     just after the matching item; otherwise, the iterator is
     positioned at the back of the container.
+//! [findNext]
 
     \sa findPrevious()
 */
 
+/*! \fn template <class T> bool QSetIterator<T>::findNext(const T &value)
+    \include qiterator.qdoc findNext
+*/
+
 /*! \fn template <class T> bool QListIterator<T>::findPrevious(const T &value)
     \fn template <class T> bool QMutableListIterator<T>::findPrevious(const T &value)