doc: Set fconf as experimental feature

Following the messages on the mailing list regarding the possible issue around
reading DTB's information, we decided to flag the fconf feature as experimental.
A uniform approach should be used to handle properties miss and DTB validation.

Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>
Change-Id: Ib3c86e81fb2e89452c593f68d825d3d8f505e1fb
diff --git a/docs/about/features.rst b/docs/about/features.rst
index 5895472..baec52b 100644
--- a/docs/about/features.rst
+++ b/docs/about/features.rst
@@ -73,6 +73,8 @@
    to be configured at runtime if required by the platform. It also enables
    loading of a hardware configuration (for example, a kernel device tree)
    as part of the FIP, to be passed through the firmware stages.
+   This feature is now incorporated inside the firmware configuration framework
+   (fconf), which is still flagged as experimental.
 
 -  Support for alternative boot flows, for example to support platforms where
    the EL3 Runtime Software is loaded using other firmware or a separate