Update oidc.md to explicitly share that sub values changed

What does this MR do and why?

Based on support tickets it was identified that users had not been aware that the sub values would change between Azure v1 and V2 providers (see !139278 (comment 1691127449)). In which case if uid_field is not set, and users do not relink their new identities, that will result in new identities being created. The doc update is intended to highlight that.

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