Require applicable cherry picks to active branches

Updates cherry pick guide to note that applicable patches must be made
to all active release branches at the same time so that the fix will be
included for each in the same round of patch releases.

Signed-off-by: hasheddan <georgedanielmangum@gmail.com>

contributors/devel/sig-release/cherry-picks.md

@@ -86,7 +86,9 @@ patch release branches.
     Please see our [recommended Git workflow](/contributors/guide/github-workflow.md#workflow).
 
   - You will need to run the cherry pick script separately for each patch
-    release you want to cherry pick to.
+    release you want to cherry pick to. Cherry picks should be applied to all
+    [active](https://github.com/kubernetes/sig-release/blob/master/releases/patch-releases.md#detailed-release-history-for-active-branches)
+    release branches where the fix is applicable.
 
 - Your cherry pick PR will immediately get the
   `do-not-merge/cherry-pick-not-approved` label.
@@ -117,6 +119,12 @@ pull requests on the `master` branch in that they:
   release branch (for example, milestone v1.11 for a cherry pick onto branch
   `release-1.11`). This is normally done for you by automation.
 
+- A separate cherry pick pull request should be open for every applicable target
+  branch. This ensures that the fix will be present on every active branch for a
+  given set of patch releases. If a fix is only applicable to a subset of active
+  branches, it is helpful to note why that is the case on the parent pull
+  request or on the cherry pick pull requests to the applicable branches.
+
 - Have one additional level of review in that they must be approved
   specifically for cherry pick by branch approvers.