Add Rhai book.

doc/src/rust/override.md

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+Override a Built-in Function
+===========================
+
+{{#include ../links.md}}
+
+Any similarly-named function defined in a script overrides any built-in or registered
+native Rust function of the same name and number of parameters.
+
+```rust
+// Override the built-in function 'to_int'
+fn to_int(num) {
+    print("Ha! Gotcha! " + num);
+}
+
+print(to_int(123));     // what happens?
+```
+
+A registered native Rust function, in turn, overrides any built-in function of the
+same name, number and types of parameters.