Update optional chaining documentation with deletion change (#2145)

* docs: add documentation regarding deletion and optional chainging

Prior to babel version 7.8, deleting a property with a nullish base
returned undefined. The default behavior in 7.8+ returns true.

* docs: Update "delete" verb tense for consistency

* Update plugin-proposal-optional-chaining.md

Co-authored-by: Nicolò Ribaudo <nicolo.ribaudo@gmail.com>

docs/plugin-proposal-optional-chaining.md

@@ -86,6 +86,18 @@ new Test?.(); // test instance
 new exists?.(); // undefined
 ```
 
+### Deleting deeply nested properties
+
+```js
+const obj = {
+  foo: {
+    bar: {}
+  },
+};
+
+const ret = delete obj?.foo?.bar?.baz; // true
+```
+
 ## Installation
 
 ```sh