Add constant NO_POS.

doc/src/plugins/function.md

@@ -117,7 +117,7 @@ pub fn greet(context: NativeCallContext, callback: FnPtr)
 The native call context is also useful in another scenario: protecting a function from malicious scripts.
 
 ```rust
-use rhai::{Dynamic, Array, NativeCallContext, EvalAltResult, Position};
+use rhai::{Dynamic, Array, NativeCallContext, EvalAltResult, NO_POS};
 use rhai::plugin::*;        // a "prelude" import for macros
 
 // This function builds an array of arbitrary size, but is protected
@@ -137,7 +137,7 @@ pub fn grow(context: NativeCallContext, size: i64)
             "Size to grow".to_string(),
             context.engine().max_array_size(),
             size as usize,
-            Position::none(),
+            NO_POS,
         ).into();
     }