Docs: Warn about thoughtless approving of risky changes

UPGRADE-4.0.md

@@ -85,7 +85,20 @@ $ vendor/bin/ecs check --ansi src/ tests/ # old
 $ vendor/bin/ecs check --ansi # new
 ```
 
-### 5. Sanity check
+### 5. BE CAREFUL WITH SUGGESTED CHANGES! ⚠️
+
+Some of the new default fixers introduced in php-coding-standard 4.0 and 4.1 suggest changes, which - if not
+thoughtfully reviewed - can change the code behavior. Especially changes introduced by (but not limited to!):
+
+- PhpdocToPropertyTypeFixer + PropertyTypeHintSniff
+- PhpdocToParamTypeFixer + ParameterTypeHintSniff
+- PhpdocToReturnTypeFixer + ReturnTypeHintSniff
+
+**Always carefully review the changes suggested by all fixers!** You may want to skip some of the checks
+(using `withSkip()`) in the first phase of upgrading to the new version of the coding standard
+or you can introduce some of the rules gradually or on a file-by-file basis.
+
+### 6. Sanity check
 Besides running your code style checks, you can ensure all predefined checks are loaded as well, by running:
 
 ```sh