github-management: clarify sponsorship criteria

An approver/reviewer in @kubernetes, may sponsor someone for the
kubernetes org or any of the related organizations
(kubernetes-sigs, kubernetes-csi etc); as long as it's a project
they're involved with.

However, this does not work the other way around.
A sponsor that is only an approver/reviewer in kubernetes-sigs cannot
sponsor someone for membership in the kubernetes org.
They are scoped just to the org they're associated with.

community-membership.md

@@ -50,8 +50,8 @@ remain active contributors to the community.
 - Sponsored by 2 reviewers. **Note the following requirements for sponsors**:
     - Sponsors must have close interactions with the prospective member - e.g. code/design/proposal review, coordinating
       on issues, etc.
-    - Sponsors must be reviewers or approvers in at least 1 OWNERS file (in any repo in the Kubernetes GitHub
-      organization)
+    - Sponsors must be reviewers or approvers in at least 1 OWNERS file either in any repo in the [Kubernetes org],
+    or the org they are sponsoring for.
     - Sponsors must be from multiple member companies to demonstrate integration across community.
 - **[Open an issue][membership request] against the kubernetes/org repo**
    - Ensure your sponsors are @mentioned on the issue

github-management/new-membership-procedure.md

@@ -51,8 +51,8 @@ required to be eligible to sponsor a new member. These requirements are:
   then the sponsor should be a member of either that org, or main Kubernetes
   org (as members of the main org have implicit membership in other orgs).
 
-- Sponsors must be a reviewer or approver in at least one OWNERS file in any
-  Kubernetes GitHub org.
+- Sponsors must be a reviewer or approver in at least one OWNERS file in
+  either the [Kubernetes GitHub org] or the org they are sponsoring for.
 
 - Sponsors must be from multiple member companies to demonstrate integration
   across community
@@ -101,3 +101,4 @@ merge.
 [community membership]: /community-membership.md
 [k-dev]: https://groups.google.com/forum/#!forum/kubernetes-dev
 [kubernetes/org]: https://git.k8s.io/org/
+[Kubernetes GitHub org]: https://github.com/kubernetes