doc: application: added description of WARN_EXPERIMENTAL setting

This commit adds description of the `WARN_EXPERIMENTAL` Kconfig setting
to the application development guide.

It describes the use of `CONFIG_WARN_EXPERIMENTAL` along with an
example of the warning output when experimental features are enabled.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

doc/application/index.rst

@@ -605,6 +605,28 @@ The other pages in the :ref:`Kconfig section of the manual <kconfig>` are also
 worth going through, especially if you planning to add new configuration
 options.
 
+Experimental features
+*********************
+
+Zephyr is a project under constant development and thus there are features that
+are still in early stages of their development cycle. Such features will be
+marked ``[EXPERIMENTAL]`` in their Kconfig title.
+
+The :kconfig:`CONFIG_WARN_EXPERIMENTAL` setting can be used to enable warnings
+at CMake configure time if any experimental feature is enabled.
+
+.. code-block:: none
+
+   CONFIG_WARN_EXPERIMENTAL=y
+
+For example, enabling experimental warnings and building a sample which enables
+:kconfig:`CONFIG_BT_EXT_ADV` will print the following warning at CMake
+configure time.
+
+.. code-block:: shell
+
+   warning: Experimental symbol BT_EXT_ADV is enabled.
+
 Devicetree Overlays
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