Update select-domain-to-use-for-email-from-microsoft-365-products.md

[Stealth7777]

Updated to explicitly call out that the tenants own domain would be used as support ask this question even though it is called out multiple times

[learn-build-service-prod]

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[v-mathavale]

Contacted the author on teams to find out if the changes are still relevant. Waiting for his reply.

[v-mathavale]

No reply from author hence closing.