Re #6156 Warn about Windows installer and long PATH

Author: mpilgrem

doc/README.md

@@ -131,6 +131,13 @@ Windows.
     use GHCup) is to download and install the
     [Windows installer](https://get.haskellstack.org/stable/windows-x86_64-installer.exe).
 
+    !!! warning
+
+        The Windows installer for Stack 2.9.1, 2.9.3 and 2.11.1 (current) (only)
+        will replace the user `PATH` environment variable (rather than append to
+        it) if a 1024 character limit is exceeded. If the content of your
+        existing user `PATH` is long, preserve it before running the installer.
+
     !!! note
 
         Systems with antivirus software may need to add Stack to the list of

doc/install_and_upgrade.md

@@ -431,6 +431,13 @@ GitHub repository.
     use GHCup) is to download and use the
     [Windows installer](https://get.haskellstack.org/stable/windows-x86_64-installer.exe).
 
+    !!! warning "Long user PATH environment variable"
+
+        The Windows installer for Stack 2.9.1, 2.9.3 and 2.11.1 (current) (only)
+        will replace the user `PATH` environment variable (rather than append to
+        it) if a 1024 character limit is exceeded. If the content of your
+        existing user `PATH` is long, preserve it before running the installer.
+
     !!! note "Anti-virus software"
 
         Systems with antivirus software may need to add Stack to the list of