x/oauth2: clarify that ExpiresIn is not automatically populated by the package

[soh335]

Added a note to the ExpiresIn field documentation to clarify that the oauth2 package does not automatically populate the ExpiresIn field, even if the server returns an "expires_in" value. This change helps prevent confusion for developers expecting ExpiresInto be set when interacting with methods likeConfig.Exchange`.

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Patch Set 1:

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[peick]

This change may get obsoleted by #748