evas: add a missing description for a newly added parameter

Summary:
A new parameter "width_offset" was added to
evas_common_font_query_last_up_to_pos() internal function.
But, internal documentation was not updated.
So, it adds a simple description for the new parameter.

Test Plan: N/A

Reviewers: jpeg, cedric, herdsman, shilpasingh

Differential Revision: https://phab.enlightenment.org/D5035

src/lib/evas/common/evas_font_query.c

@@ -816,6 +816,7 @@ end:
  * @param text_props the string object.
  * @param x the x boundary.
  * @param y the y boundary.
+ * @param width_offset the additional width only for allowing glyph's.
  * @return the position found, -1 on failure.
  */
 EAPI int