Revise docs.

doc/src/language/constants.md

@@ -16,8 +16,45 @@ x = 123;            // <- syntax error: cannot assign to constant
 ```
 
 Unlike variables which need not have initial values (default to [`()`]),
-constants must be assigned one, and it must be a constant _value_, not an expression.
+constants must be assigned one, and it must be a [_literal value_](../appendix/literals.md),
+not an expression.
 
 ```rust
 const x = 40 + 2;   // <- syntax error: cannot assign expression to constant
 ```
+
+
+Manually Add Constant into Custom Scope
+--------------------------------------
+
+It is possible to add a constant into a custom [`Scope`] so it'll be available to scripts
+running with that [`Scope`].
+
+When added to a custom [`Scope`], a constant can hold any value, not just a literal value.
+
+It is very useful to have a constant value hold a [custom type], which essentially acts
+as a [_singleton_](../patterns/singleton.md).  The singleton object can be modified via its
+registered API - being a constant only prevents it from being re-assigned or operated upon by Rhai;
+mutating it via a Rust function is still allowed.
+
+```rust
+use rhai::{Engine, Scope};
+
+struct TestStruct(i64);                                     // custom type
+
+let engine = Engine::new()
+    .register_type_with_name::<TestStruct>("TestStruct")    // register custom type
+    .register_get_set("value",
+        |obj: &mut TestStruct| obj.0,                       // property getter
+        |obj: &mut TestStruct, value: i64| obj.0 = value    // property setter
+    );
+
+let mut scope = Scope::new();                               // create custom scope
+
+scope.push_constant("MY_NUMBER", TestStruct(123_i64));      // add constant variable
+
+engine.consume_with_scope(&mut scope, r"
+    MY_NUMBER.value = 42;                                   // constant objects can be modified
+    print(MY_NUMBER.value);                                 // prints 42
+")?;
+```