Fix '1' + 2n BigInt example on Addition page (mdn#43812)

* Fix '1' + 2n BigInt example on Addition page

The 'Addition using BigInts' example marked "1" + 2n as throwing a
TypeError, but the + operator checks for a string operand before the
BigInt/Number mix check, so "1" + 2n performs string concatenation
and returns "12". Remove the incorrect line and add a short clarifying
section with the real behavior.

Fixes mdn#43807

* Update index.md

---------

Co-authored-by: Joshua Chen <sidachen2003@gmail.com>

Author: tmchow

files/en-us/web/javascript/reference/operators/addition/index.md

@@ -70,12 +70,17 @@ false + false; // 0
 1n + 2n; // 3n
 ```
 
-You cannot mix BigInt and number operands in addition.
+You cannot mix BigInt and number operands in addition. `null`, `undefined`, and boolean values are coerced to numbers and are forbidden as well.
 
 ```js example-bad
 1n + 2; // TypeError: Cannot mix BigInt and other types, use explicit conversions
 2 + 1n; // TypeError: Cannot mix BigInt and other types, use explicit conversions
-"1" + 2n; // TypeError: Cannot mix BigInt and other types, use explicit conversions
+```
+
+Strings have priority over over types, so adding a string to a BigInt produces string concatenation rather than a `TypeError`.
+
+```js
+"1" + 2n; // "12"
 ```
 
 To do addition with a BigInt and a non-BigInt, convert either operand: