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.

release/scripts/startup/bl_ui/space_view3d.py

@@ -6227,17 +6227,20 @@ class VIEW3D_PT_overlay_edit_mesh(Panel):
         overlay = view.overlay
         display_all = overlay.show_overlays
 
+        is_any_solid_shading = not (shading.show_xray or (shading.type == 'WIREFRAME'))
+
         col = layout.column()
         col.active = display_all
 
         split = col.split()
 
         sub = split.column()
-        sub.active = not ((shading.type == 'WIREFRAME') or shading.show_xray)
+        sub.active = is_any_solid_shading
         sub.prop(overlay, "show_edges", text="Edges")
         sub = split.column()
         sub.prop(overlay, "show_faces", text="Faces")
         sub = split.column()
+        sub.active = is_any_solid_shading
         sub.prop(overlay, "show_face_center", text="Center")
 
         row = col.row(align=True)

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);