minor symfony#26276 [Doc] Add entry for container.dumper.inline_class_loader param at UPGRADE-3.4.md and UPGRADE-4.0.md (phansys)

This PR was merged into the 3.4 branch.

Discussion
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[Doc] Add entry for `container.dumper.inline_class_loader` param at `UPGRADE-3.4.md` and `UPGRADE-4.0.md`

|Q            |A     |
|---          |---   |
|Branch       |3.4   |
|Bug fix?     |no    |
|New feature? |no    |
|BC breaks?   |no    |
|Deprecations?|no    |
|Tests pass?  |yes   |
|Fixed tickets|symfony#26269|
|License      |MIT   |
|Doc PR       |n/a   |

Commits
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d6e2b81 Add entry for `container.dumper.inline_class_loader` param at `UPGRADE-3.4.md` and `UPGRADE-4.0.md`

Author: nicolas-grekas

UPGRADE-3.4.md

@@ -72,6 +72,16 @@ DependencyInjection
  * The `ResolveDefinitionTemplatesPass` class is deprecated and will be removed in 4.0.
    Use the `ResolveChildDefinitionsPass` class instead.
 
+ * Unless you're using a custom autoloader, you should enable the `container.dumper.inline_class_loader`
+   parameter. This can drastically improve DX by reducing the time to load classes
+   when the `DebugClassLoader` is enabled. If you're using `FrameworkBundle`, this
+   performance improvement will also impact the "dev" environment:
+
+   ```yml
+   parameters:
+       container.dumper.inline_class_loader: true
+   ```
+
 Debug
 -----
 

UPGRADE-4.0.md

@@ -236,6 +236,17 @@ DependencyInjection
 
  * The `ExtensionCompilerPass` has been moved to before-optimization passes with priority -1000.
 
+ * In 3.4, parameter `container.dumper.inline_class_loader` was introduced. Unless
+   you're using a custom autoloader, you should enable this parameter. This can
+   drastically improve DX by reducing the time to load classes when the `DebugClassLoader`
+   is enabled. If you're using `FrameworkBundle`, this performance improvement will
+   also impact the "dev" environment:
+
+   ```yml
+   parameters:
+       container.dumper.inline_class_loader: true
+   ```
+
 DoctrineBridge
 --------------