device property: Fix documentation for *_match_string() APIs

The returned value on success is an index of the matching string,
starting from 0. Reflect this in the documentation.

Fixes: 3f5c8d3 ("device property: Add fwnode_property_match_string()")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: andy-shev

drivers/base/property.c

@@ -229,7 +229,7 @@ EXPORT_SYMBOL_GPL(device_property_read_string);
  * Find a given string in a string array and if it is found return the
  * index back.
  *
- * Return: %0 if the property was found (success),
+ * Return: index, starting from %0, if the property was found (success),
  *	   %-EINVAL if given arguments are not valid,
  *	   %-ENODATA if the property does not have a value,
  *	   %-EPROTO if the property is not an array of strings,
@@ -450,7 +450,7 @@ EXPORT_SYMBOL_GPL(fwnode_property_read_string);
  * Find a given string in a string array and if it is found return the
  * index back.
  *
- * Return: %0 if the property was found (success),
+ * Return: index, starting from %0, if the property was found (success),
  *	   %-EINVAL if given arguments are not valid,
  *	   %-ENODATA if the property does not have a value,
  *	   %-EPROTO if the property is not an array of strings,