Add usage string

Author: rocky

mathics/autoload/settings.m

@@ -11,6 +11,8 @@
 Settings`$PreferredBackendMethod = "sympy"
 Unprotect[Settings`$PreferredBackendMethod]
 
-(*Needed for Feyncalc*)
+(* Some packages like Feyncalc, test for whether a they are being used
+inside a notbook. *)
+System`$Notebooks::usage = "Set True if the Mathics is being used with a notebook‐based front end.";
 System`$Notebooks = False;
 Unprotect[System`Notebooks];