Add Rhai book.

doc/src/language/overload.md

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+Function Overloading
+===================
+
+{{#include ../links.md}}
+
+Functions defined in script can be _overloaded_ by _arity_ (i.e. they are resolved purely upon the function's _name_
+and _number_ of parameters, but not parameter _types_ since all parameters are the same type - [`Dynamic`]).
+
+New definitions _overwrite_ previous definitions of the same name and number of parameters.
+
+```rust
+fn foo(x,y,z) { print("Three!!! " + x + "," + y + "," + z) }
+fn foo(x) { print("One! " + x) }
+fn foo(x,y) { print("Two! " + x + "," + y) }
+fn foo() { print("None.") }
+fn foo(x) { print("HA! NEW ONE! " + x) }    // overwrites previous definition
+
+foo(1,2,3);     // prints "Three!!! 1,2,3"
+foo(42);        // prints "HA! NEW ONE! 42"
+foo(1,2);       // prints "Two!! 1,2"
+foo();          // prints "None."
+```