bump cortex-m-rt version to v0.2.3; document the _stext symbol

memory.x

@@ -10,3 +10,10 @@ MEMORY
 /* The stack is of the full descending type. */
 /* NOTE Do NOT modify `_stack_start` unless you know what you are doing */
 _stack_start = ORIGIN(RAM) + LENGTH(RAM);
+
+/* You can use this symbol to customize the location of the .text section */
+/* If omitted the .text section will be placed right after the .vector_table
+   section */
+/* This is required only on some microcontrollers that store some configuration
+   right after the vector table */
+/* _stext = ORIGIN(FLASH) + 0x400; */