docs: Add reference for Thumb2 inline assembler.

Thanks to Peter Hinch for contributing this.

docs/reference/asm_thumb2_stack.rst

@@ -0,0 +1,20 @@
+Stack push and pop
+==================
+
+Document conventions
+--------------------
+
+The ``push()`` and ``pop()`` instructions accept as their argument a register set containing
+a subset, or possibly all, of the general-purpose registers R0-R12 and the link register (lr or R14).
+As with any Python set the order in which the registers are specified is immaterial. Thus the
+in the following example the pop() instruction would restore R1, R7 and R8 to their contents prior
+to the push():
+
+* push({r1, r8, r7}) Save three registers on the stack.
+* pop({r7, r1, r8}) Restore them
+
+Stack operations
+----------------
+
+* push({regset}) Push a set of registers onto the stack
+* pop({regset}) Restore a set of registers from the stack