dm: core: Fix up string-function documentation

The details for of_property_read_string_helper() and
ofnode_read_string_index() are a little inaccurate. Fix up the comments to
avoid confusion.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/core/of_access.c b/drivers/core/of_access.c
index 9960e6b..3707143 100644
--- a/drivers/core/of_access.c
+++ b/drivers/core/of_access.c
@@ -581,7 +581,8 @@
  * @propname:	name of the property to be searched.
  * @out_strs:	output array of string pointers.
  * @sz:		number of array elements to read.
- * @skip:	Number of strings to skip over at beginning of list.
+ * @skip:	Number of strings to skip over at beginning of list (cannot be
+ *	negative)
  *
  * Don't call this function directly. It is a utility helper for the
  * of_property_read_string*() family of functions.