UI: Improve description of custom asset library type

Description added in 9c2330d821.

It is unclear what "external" in this case means, why these asset
libraries are considered "normal" (are the others not "normal"?) and the
term "asset repository" is wrong (should be "asset library").

I think this isn't actually shown in the UI currently in fact.

source/blender/makesrna/intern/rna_asset.cc

@@ -31,7 +31,11 @@ const EnumPropertyItem rna_enum_asset_library_type_items[] = {
      0,
      "Essentials",
      "Show the basic building blocks and utilities coming with Blender"},
-    {ASSET_LIBRARY_CUSTOM, "CUSTOM", 0, "Custom", "All external 'normal' asset repositories"},
+    {ASSET_LIBRARY_CUSTOM,
+     "CUSTOM",
+     0,
+     "Custom",
+     "Show assets from the asset libraries configured in the Preferences"},
     {0, nullptr, 0, nullptr, nullptr},
 };