[SSL] Clarify same CSR vs new CSR and cert upload (#18984)

* Update certificate-signing-requests.mdx

* Apply suggestion from code review

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Co-authored-by: Rebecca Tamachiro <[email protected]>

Author: ngayerie

src/content/docs/ssl/edge-certificates/additional-options/certificate-signing-requests.mdx

@@ -46,3 +46,5 @@ To use a CSR:
 ## Renew a certificate
 
 When you [renew a custom certificate](/ssl/edge-certificates/custom-certificates/renewing/), you can reuse a previously generated CSR.
+
+Note that it is not possible to use a different CSR with the same certificate. In this case, you must upload the certificate as a new certificate, selecting the new CSR.