[doc][ybm][faq] Add the reasoning behind not providing access to yugabyte user in YBM

[mrajcevic01]

Description

A common question many YBM users have is why do they not have access to the yugabyte user. This could be unexpected as they do have access to this user in open-source YugabyteDB. We do provide insight into this in a couple of places

When creating a cluster:

For security reasons, the database admin user does not have YSQL superuser privileges, but does have sufficient privileges for most tasks. For more information on database roles and privileges in YugabyteDB Managed, refer to Database authorization in YugabyteDB Managed clusters.

As well as in our Database Authorization section.

However this is met often enough to call this out in our FAQ for further visibility and for SEO purposes (if a user were to search for this)

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