Update user-get-mailboxsettings.md

[KoenZomers]

Adding notice that mailboxSettings through /users/ only works with an application context

[learn-build-service-prod[bot]]

Learn Build status updates of commit 4879c1c:

:white_check_mark: Validation status: passed

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api-reference/v1.0/api/user-get-mailboxsettings.md	:white_check_mark:Succeeded

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[FaithOmbongi]

Hi @SuryaLashmiS, can you validate this change? Thanks.

[Lauragra]

@SuryaLashmiS , can you please review this PR or let us know if we can close it?