doc: add two diagrams to user mode documentation

One shows how globals are routed for automatic memory
domains. The other illustrates control flow when making
system calls.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/reference/usermode/memory_domain.rst

@@ -213,6 +213,12 @@ contiguous block of memory, zero any BSS variables at boot, and define
 a memory partition of appropriate base address and size which contains all
 the tagged data.
 
+.. figure:: auto_mem_domain.png
+   :alt: Automatic Memory Domain build flow
+   :align: center
+
+   Automatic Memory Domain build flow
+
 Automatic memory partitions are only configured as read-write
 regions. They are defined with :c:macro:`K_APPMEM_PARTITION_DEFINE()`.
 Global variables are then routed to this partition using

doc/reference/usermode/syscalls.rst

@@ -170,6 +170,13 @@ necessary privilege elevation. In this layer, all arguments are treated as an
 unsigned 32-bit type. There is always a 32-bit unsigned return value, which
 may or may not be used.
 
+.. figure:: syscall_flow.png
+   :alt: System Call execution flow
+   :width: 80%
+   :align: center
+
+   System Call execution flow
+
 Some system calls may have more than six arguments. The number of
 arguments passed via registers is limited to six for all
 architectures. Additional arguments will need to be passed in an array