docs: update docs with new RXTX_MAP/UNMAP behavior

Update the SPM docs to mention that RXTX_MAP will now unmap the
send/recv pages NWd page tables.

Change-Id: Ib20c37c347f19300f5ab57dfd4b7a2b571b40725
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/docs/secure-partition-manager/secure-partition-manager.rst b/docs/secure-partition-manager/secure-partition-manager.rst
index 3115f8e..f946d91 100644
--- a/docs/secure-partition-manager/secure-partition-manager.rst
+++ b/docs/secure-partition-manager/secure-partition-manager.rst
@@ -688,11 +688,13 @@
 which is expected to receive messages from the secure world. The SPMC will in
 this case allocate internal state structures to facilitate RX buffer access
 synchronization (through FFA_RX_ACQUIRE interface), and to permit SPs to send
-messages.
+messages. The addresses used must be contained in the SPMC manifest NS memory
+node (see `SPMC manifest`_).
 
 The FFA_RXTX_UNMAP unmaps the RX/TX pair from the translation regime of the
 caller, either it being the Hypervisor or OS kernel, as well as a secure
-partition.
+partition, and restores them in the VM's translation regime so that they can be
+used for memory sharing operations from the normal world again.
 
 FFA_PARTITION_INFO_GET
 ~~~~~~~~~~~~~~~~~~~~~~