doc: fix vm.Script createCachedData example

`Script.createCachedData` and `SourceTextModule.createCachedData`
doesn't serialize JavaScript variables.

doc/api/vm.md

@@ -126,6 +126,15 @@ Creates a code cache that can be used with the `Script` constructor's
 `cachedData` option. Returns a `Buffer`. This method may be called at any
 time and any number of times.
 
+The code cache of the `Script` doesn't contain any JavaScript observable
+states. The code cache is safe to be saved along side the script source and
+used to construct new `Script` instances multiple times.
+
+Functions in the `Script` source can be marked as lazily compiled and they are
+not compiled at construction of the `Script`. These functions are forced to be
+compiled when they are invoked the first time. The code cache serializes the
+current compilation state.
+
 ```js
 const script = new vm.Script(`
 function add(a, b) {
@@ -135,11 +144,13 @@ function add(a, b) {
 const x = add(1, 2);
 `);
 
-const cacheWithoutX = script.createCachedData();
+const cache1 = script.createCachedData();
+// cached1 contains function `add` with lazy compilation stub.
 
 script.runInThisContext();
 
-const cacheWithX = script.createCachedData();
+const cache2 = script.createCachedData();
+// cached2 contains compiled function `add`.
 ```
 
 ### `script.runInContext(contextifiedObject[, options])`
@@ -780,6 +791,15 @@ Creates a code cache that can be used with the `SourceTextModule` constructor's
 `cachedData` option. Returns a `Buffer`. This method may be called any number
 of times before the module has been evaluated.
 
+The code cache of the `SourceTextModule` doesn't contain any JavaScript
+observable states. The code cache is safe to be saved along side the script
+source and used to construct new `SourceTextModule` instances multiple times.
+
+Functions in the `SourceTextModule` source can be marked as lazily compiled and
+they are not compiled at construction of the `SourceTextModule`. These
+functions are forced to be compiled when they are invoked the first time. The
+code cache serializes the current compilation state.
+
 ```js
 // Create an initial module
 const module = new vm.SourceTextModule('const a = 1;');