Add CoglMaterialWrapMode to the Cogl docs and fix docs for the getters

CoglMaterialWrapMode was missing from the cogl-sections.txt file so it
wasn't getting displayed. There were also no documented return values
from the getters.

cogl/cogl-material.h

@@ -942,6 +942,8 @@ cogl_material_set_layer_wrap_mode (CoglMaterial        *material,
  *
  * Gets the wrap mode for the 's' coordinate of texture lookups on this layer.
  *
+ * Return value: the wrap mode value for the s coordinate.
+ *
  * Since: 1.4
  */
 CoglMaterialWrapMode
@@ -953,6 +955,8 @@ cogl_material_layer_get_wrap_mode_s (CoglMaterialLayer *layer);
  *
  * Gets the wrap mode for the 't' coordinate of texture lookups on this layer.
  *
+ * Return value: the wrap mode value for the t coordinate.
+ *
  * Since: 1.4
  */
 CoglMaterialWrapMode

doc/reference/cogl/cogl-sections.txt

@@ -496,6 +496,7 @@ cogl_material_layer_get_type
 cogl_material_layer_get_texture
 cogl_material_layer_get_min_filter
 cogl_material_layer_get_mag_filter
+CoglMaterialWrapMode
 cogl_material_set_layer_wrap_mode
 cogl_material_set_layer_wrap_mode_s
 cogl_material_set_layer_wrap_mode_t