add comment explaining things

* removal of bsd tar 614bdc0
* garden-runc release where things were fixed: https://github.com/cloudfoundry/garden-runc-release/releases/tag/v1.76.0

Author: KauzClay

modules/BOSH.Agent/BOSH.Agent.psm1

@@ -68,6 +68,11 @@ function Copy-Agent
     Move-Item (Join-Path $boshDir (Join-Path "deps" "*")) $depsDir
     Remove-Item -Path (Join-Path $boshDir "deps") -Force
 
+    # Although we previously removed bsdtar in favor of the system tar that
+    # comes with windows, some older garden-runc releases (< v1.76.0) still
+    # expect tar to live there. This preserves backwards compatability. We can
+    # likely rip this out again once we're out of some reasonable window where
+    # someone will still be running an older garden-runc release
     Copy-Item "C:\Windows\system32\tar.exe" -Destination (Join-Path $depsDir "tar.exe")
 }