docs/library/machine: Add machine.memX to docs with brief example.

docs/library/machine.rst

@@ -19,6 +19,44 @@ This is true for both physical devices with IDs >= 0 and "virtual" devices
 with negative IDs like -1 (these "virtual" devices are still thin shims on
 top of real hardware and real hardware interrupts). See :ref:`isr_rules`.
 
+Memory access
+-------------
+
+The module exposes three objects used for raw memory access.
+
+.. data:: mem8
+
+    Read/write 8 bits of memory.
+
+.. data:: mem16
+
+    Read/write 16 bits of memory.
+
+.. data:: mem32
+
+    Read/write 32 bits of memory.
+
+Use subscript notation ``[...]`` to index these objects with the address of
+interest. Note that the address is the byte address, regardless of the size of
+memory being accessed.
+
+Example use (registers are specific to an stm32 microcontroller):
+
+.. code-block:: python3
+
+    import machine
+    from micropython import const
+
+    GPIOA = const(0x48000000)
+    GPIO_BSRR = const(0x18)
+    GPIO_IDR = const(0x10)
+
+    # set PA2 high
+    machine.mem32[GPIOA + GPIO_BSRR] = 1 << 2
+
+    # read PA3
+    value = (machine.mem32[GPIOA + GPIO_IDR] >> 3) & 1
+
 Reset related functions
 -----------------------