Update spec for push and pop operations

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

vm.md

@@ -200,14 +200,21 @@ wrapping around to 0.
 * Push
     * Opcode: 0x2002
     * Params: Source
+    * When this instruction is executed, these actions occur:
+        * Set the value in memory at the current stack pointer to the source value.
+        * Increment the stack pointer by the size of value at the source.
 * Pop
     * Opcode: 0x2003
     * Params: Dest
+    * When this instruction is executed, these actions occur:
+        * Decrement the stack pointer by the size of value at the destination.
+        * Copy the value at the stack pointer into the destination.
 
 ## Data movement
 
 * Mov
     * Opcode: 0x3000
+    * Params: Source, Dest
 
 ## Miscellaneous