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/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 778b49c..90fe83f 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -643,6 +643,7 @@
 -  ``ARM_IO_IN_DTB``: This flag determines whether to use IO based on the
    firmware configuration framework. This will move the io_policies into a
    configuration device tree, instead of static structure in the code base.
+   This is currently an experimental feature.
 
 
 -  ``USE_ROMLIB``: This flag determines whether library at ROM will be used.