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>