architecture: core: detail reserved/dynamic shared memory

Detail a bit contiguous/reserved and dynamic shared memory are and
highlight the latter is preferred on supported platforms.

Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
diff --git a/architecture/core.rst b/architecture/core.rst
index 6295e49..c259942 100644
--- a/architecture/core.rst
+++ b/architecture/core.rst
@@ -1417,6 +1417,12 @@
 of shared memory areas: an area for contiguous buffers and an area for
 noncontiguous buffers. At least one has to be enabled.
 
+Contiguous shared memory is the historical OP-TEE legacy shared memory
+scheme where a specific physical memory area is shared. Nowadays,
+platforms tend to describe the physical memory layout and enable
+noncontiguous dynamic shared memory, allowing the non-secure OS to
+use its native system memory as legitimate shared memory references.
+
 Contiguous shared buffers
 =========================
 Configuration directives ``CFG_SHMEM_START`` and ``CFG_SHMEM_SIZE``