[docs] Add QSpan details to overview of C++20 features

Document that QSpan exists because we cannot use std::span, and
explicitly point to the differences between QSpan and std::span.

Task-number: QTBUG-128837
Change-Id: I54f406e3306ee1da136107323887f0c87f94ff9b
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit 083ebfa1a5cb16b0b62ae5b5855a602b2f9ba818)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 8bd0625f54bf7cbd2121bbdbcb45fd5f9566b63f)

src/corelib/doc/src/cpp20-overview.qdoc

@@ -31,4 +31,20 @@
     {https://en.cppreference.com/w/cpp/chrono/duration} {duration
     types}. QTimeZone now supports construction from a \l
     {https://en.cppreference.com/w/cpp/chrono/time_zone} {time_zone}.
+
+    \section1 QSpan - a Qt version of \c {std::span}
+
+    The \l {https://en.cppreference.com/w/cpp/container/span}{std::span} class
+    template is an addition to the standard library that provides a uniform
+    way to access a contiguous portion of any contiguous container.
+
+    Using \c {std::span} in the public APIs of Qt could provide significant
+    advantages. However, Qt 6 only requires C++17, meaning it cannot have C++20
+    types in the API and ABI.
+
+    Qt 6.7 introduced \l QSpan - a Qt version of \c {std::span}. The API of the
+    class is compatible with the std version. \l QSpan and \c {std::span} can be
+    implicitly converted into each other. However, there are some differences
+    between the classes. See the corresponding section in the
+    \l {span-STL} {QSpan class documentation} for more details.
 */