Clarify the JSON data referenced in patterns sample is deserialized (#…

…6422)

1. Renamed the variable json to data throughout the section.
2. Updated the text to clarify that the example works with deserialized
data (e.g., parsed from JSON) rather than raw JSON strings.
3. Ensured consistency in variable naming and explanations.

This update makes the documentation more accurate and avoids misleading
readers into thinking that the variable holds a JSON string.

Resolves #6411

---------

Co-authored-by: Parker Lougheed <parlough@gmail.com>

examples/language/lib/patterns/json.dart

@@ -2,18 +2,18 @@
 
 void main() {
   // #docregion json-1
-  var json = {
+  var data = {
     'user': ['Lily', 13],
   };
-  var {'user': [name, age]} = json;
+  var {'user': [name, age]} = data;
   // #enddocregion json-1
 
   {
     // #docregion json-2
-    if (json is Map<String, Object?> &&
-        json.length == 1 &&
-        json.containsKey('user')) {
-      var user = json['user'];
+    if (data is Map<String, Object?> &&
+        data.length == 1 &&
+        data.containsKey('user')) {
+      var user = data['user'];
       if (user is List<Object> &&
           user.length == 2 &&
           user[0] is String &&
@@ -27,7 +27,7 @@ void main() {
   }
   {
     // #docregion json-3
-    if (json case {'user': [String name, int age]}) {
+    if (data case {'user': [String name, int age]}) {
       print('User $name is $age years old.');
     }
     // #enddocregion json-3

src/content/language/patterns.md

@@ -360,14 +360,14 @@ double calculateArea(Shape shape) => switch (shape) {
 ### Validating incoming JSON
 
 [Map][] and [list][] patterns work well for destructuring key-value pairs in
-JSON data:
+deserialized data, such as data parsed from JSON:
 
 <?code-excerpt "language/lib/patterns/json.dart (json-1)"?>
 ```dart 
-var json = {
+var data = {
   'user': ['Lily', 13],
 };
-var {'user': [name, age]} = json;
+var {'user': [name, age]} = data;
 ```
 
 If you know that the JSON data has the structure you expect,
@@ -379,10 +379,10 @@ Without patterns, validation is verbose:
 
 <?code-excerpt "language/lib/patterns/json.dart (json-2)"?>
 ```dart
-if (json is Map<String, Object?> &&
-    json.length == 1 &&
-    json.containsKey('user')) {
-  var user = json['user'];
+if (data is Map<String, Object?> &&
+    data.length == 1 &&
+    data.containsKey('user')) {
+  var user = data['user'];
   if (user is List<Object> &&
       user.length == 2 &&
       user[0] is String &&
@@ -402,7 +402,7 @@ method of validating JSON:
 
 <?code-excerpt "language/lib/patterns/json.dart (json-3)"?>
 ```dart
-if (json case {'user': [String name, int age]}) {
+if (data case {'user': [String name, int age]}) {
   print('User $name is $age years old.');
 }
 ```