fix: remove "experimental" tag for stable features

there are features which are marked as experimental even though they
are stable and used for quite some time.
Following features are no longer marked as experimental
  - SPMD
  - MEASURED_BOOT
  - FCONF and associated build flags
  - DECRYPTION_SUPPORT and associated build flags
  - ENABLE_PAUTH
  - ENABLE_BTI
  - USE_SPINLOCK_CAS
  - GICv3 Multichip support

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I4bb653d9c413c66095ec31f0b8aefeb13ea04ee9
diff --git a/docs/getting_started/porting-guide.rst b/docs/getting_started/porting-guide.rst
index 6569a47..57eba3c 100644
--- a/docs/getting_started/porting-guide.rst
+++ b/docs/getting_started/porting-guide.rst
@@ -891,8 +891,7 @@
 
 On success the function should return 0 and a negative error code otherwise.
 
-Note that this API depends on ``DECRYPTION_SUPPORT`` build flag which is
-marked as experimental.
+Note that this API depends on ``DECRYPTION_SUPPORT`` build flag.
 
 Function : plat_fwu_set_images_source() [when PSA_FWU_SUPPORT == 1]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~