docs: mark PSA_CRYPTO as an experimental feature

Updated the documentation to mark PSA_CRYPTO as an experimental
feature.

Change-Id: I894b687d6727fe7f80df54e6b08937e171f459b6
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 654ddc5..347cf20 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -1195,7 +1195,8 @@
   The platform will use PSA compliant Crypto APIs during authentication and
   image measurement process by enabling this option. It uses APIs defined as
   per the `PSA Crypto API specification`_. This feature is only supported if
-  using MbedTLS 3.x version. By default it is disabled (``0``).
+  using MbedTLS 3.x version. By default it is disabled (``0``), and this is an
+  experimental feature.
 
 - ``ENABLE_CONSOLE_GETC``: Boolean option to enable `getc()` feature in console
   driver(s). By default it is disabled (``0``) because it constitutes an attack