Make it clear in documentation that these methods are not implemented.

Even so return a more meaningful value - 0 is not a member of the Enum it returns.

src/lib/evas/canvas/evas_object_textgrid.c

@@ -1429,7 +1429,7 @@ EOLIAN static Evas_Textgrid_Font_Style
 _evas_textgrid_supported_font_styles_get(Eo *eo_obj EINA_UNUSED, Evas_Textgrid_Data *o EINA_UNUSED)
 {
    /* FIXME: to do */
-   return 0;
+   return EVAS_TEXTGRID_FONT_STYLE_NORMAL;
 }
 
 EOLIAN static void

src/lib/evas/canvas/evas_textgrid.eo

@@ -5,10 +5,13 @@ class Evas.Textgrid (Evas.Object, Efl.Text_Properties)
    properties {
       supported_font_styles {
          set {
-            /*@ No description supplied by the EAPI. */
+            /*@
+            This function is not implemented yet, do not use. */
          }
          get {
-            /*@ No description supplied by the EAPI. */
+            /*@
+            This function is not implemented yet, do not use.
+            @return EVAS_TEXTGRID_FONT_STYLE_NORMAL */
          }
          values {
             Evas_Textgrid_Font_Style styles;