Extend container overview documentation, mention Q_DECLARE_TYPEINFO

The container overview documentation did not mention
Q_DECLARE_TYPEINFO and the related optimizations. This patch
adds a short paragraph about it.

Task-number: QTBUG-86584
Pick-to: 6.0
Change-Id: I5b0b8ce92a47da5f0398cc413fbf3e07b0921e59
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

src/corelib/doc/src/containers.qdoc

@@ -527,6 +527,29 @@
     with the expected number of items before you insert the items.
     The next section discusses this topic in more depth.
 
+    \section1 Optimizations for Primitive and Relocatable Types
+
+    Qt containers can use optimized code paths if the stored
+    elements are relocatable or even primitive.
+    However, whether types are primitive or relocatable
+    cannot be detected in all cases.
+    You can declare your types to be primitive or relocatable
+    by using the Q_DECLARE_TYPEINFO macro with the Q_PRIMITIVE_TYPE
+    flag or the Q_RELOCATABLE_TYPE flag. See the documentation
+    of Q_DECLARE_TYPEINFO for further details and usage examples.
+
+    If you do not use Q_DECLARE_TYPEINFO,
+    Qt will use
+    \l {https://en.cppreference.com/w/cpp/types/is_trivial} {std::is_trivial_v<T>}
+    to indentify primitive
+    types and it will require both
+    \l {https://en.cppreference.com/w/cpp/types/is_trivially_copyable} {std::is_trivially_copyable_v<T>}
+    and
+    \l {https://en.cppreference.com/w/cpp/types/is_destructible} {std::is_trivially_destructible_v<T>}
+    to identify relocatable types.
+    This is always a safe choice, albeit
+    of maybe suboptimal performance.
+
     \section1 Growth Strategies
 
     QList<T>, QString, and QByteArray store their items

src/corelib/global/qglobal.cpp

@@ -3995,12 +3995,19 @@ bool qunsetenv(const char *varName)
     pattern still represents a valid object, and memcpy() can be used
     to create a valid independent copy of a QUuid object.
 
-    Example of a movable type:
+    Example of a relocatable type:
 
     \snippet code/src_corelib_global_qglobal.cpp 39
 
-    Qt will try to detect the class of a type using std::is_trivial or
-    std::is_trivially_copyable. Use this macro to tune the behavior.
+    Qt will try to detect the class of a type using
+    \l {https://en.cppreference.com/w/cpp/types/is_trivial} {std::is_trivial_v<T>}
+    to indentify primitive
+    types and it will require both
+    \l {https://en.cppreference.com/w/cpp/types/is_trivially_copyable} {std::is_trivially_copyable_v<T>}
+    and
+    \l {https://en.cppreference.com/w/cpp/types/is_destructible} {std::is_trivially_destructible_v<T>}
+    to identify relocatable types.
+    Use this macro to tune the behavior.
     For instance many types would be candidates for Q_RELOCATABLE_TYPE despite
     not being trivially-copyable.
 */