UI: clarify edit-mesh face center behavior

- Grey out in wire/xray display.
- Expand the description for when this is used.

While this is working, the intended behavior wasn't clear,
address T85177.

source/blender/makesrna/intern/rna_space.c

@@ -4020,7 +4020,10 @@ static void rna_def_space_view3d_overlay(BlenderRNA *brna)
 
   prop = RNA_def_property(srna, "show_face_center", PROP_BOOLEAN, PROP_NONE);
   RNA_def_property_boolean_sdna(prop, NULL, "overlay.edit_flag", V3D_OVERLAY_EDIT_FACE_DOT);
-  RNA_def_property_ui_text(prop, "Draw Face Center", "Display face center");
+  RNA_def_property_ui_text(
+      prop,
+      "Draw Face Center",
+      "Display face center when face selection is enabled in solid shading modes");
   RNA_def_property_update(prop, NC_SPACE | ND_SPACE_VIEW3D, NULL);
 
   prop = RNA_def_property(srna, "show_edge_crease", PROP_BOOLEAN, PROP_NONE);