Update service-limits.md

I hear from dev team that the number of recipients in email for SMTP cannot be increased more than 50 at this time (The one for REST API can be increased by the quota request).

I think that we should write the limit as a Note.

Author: atsushina

articles/communication-services/concepts/service-limits.md

@@ -185,6 +185,7 @@ To increase your email quota, follow the instructions in [Quota increase for ema
 
 > [!NOTE]
 > Email quota increase requests might take up to 72 hours for evaluation and approval, especially for requests that come in on Friday afternoon.
+> The quota increase requests for the number of recipients in email of SMTP is not supported at this time.
 
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