dm: core: Update docs about oftree_from_fdt()

Update this function's comment and also the livetree documentation, so it
is clear when to use the function.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/develop/driver-model/livetree.rst

@@ -255,7 +255,7 @@ So long as OF_LIVE is disabled, it is possible to do fixups using the ofnode
 interface. The OF_LIVE support required addition of the flattening step at the
 end.
 
-See dm_test_ofnode_root() for some examples. The ofnode_path_root() function
+See dm_test_ofnode_root() for some examples. The oftree_from_fdt() function
 causes a flat device tree to be 'registered' such that it can be used by the
 ofnode interface.
 

include/dm/ofnode.h

@@ -59,6 +59,9 @@ __attribute_const__ int ofnode_to_offset(ofnode node);
 /**
  * oftree_from_fdt() - Returns an oftree from a flat device tree pointer
  *
+ * If @fdt is not already registered in the list of current device trees, it is
+ * added to the list.
+ *
  * @fdt: Device tree to use
  *
  * Returns: reference to the given node