Clarify autolink users for SAML provider

What does this MR do?

The OmniAuth docs incorrectly suggests you can use the gitlab_rails['omniauth_auto_link_user'] setting for the SAML provider.

If a GitLab instance is using the SAML provider and if a user adds the following to their gitlab.rb file, this will not work:

gitlab_rails['omniauth_auto_link_user'] = ['saml']

This setting will work however:

gitlab_rails['omniauth_auto_link_saml_user'] = true

This discrepancy is confusing.

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Code analysis

By default, OmniAuth users are auto linked here https://gitlab.com/gitlab-org/gitlab/-/blob/v14.1.2-ee/lib/gitlab/auth/o_auth/user.rb#L75. This then calls auto_link_user? and then it looks at Gitlab.config.omniauth.auto_link_user https://gitlab.com/gitlab-org/gitlab/-/blob/v14.1.2-ee/lib/gitlab/auth/o_auth/user.rb#L286

For the SAML provider, SAML users are auto linked here https://gitlab.com/gitlab-org/gitlab/-/blob/v14.1.2-ee/lib/gitlab/auth/saml/user.rb#L21. This then calls auto_link_saml_user? (notice the different def name) and then it looks at Gitlab.config.omniauth.auto_link_saml_user https://gitlab.com/gitlab-org/gitlab/-/blob/v14.1.2-ee/lib/gitlab/auth/saml/user.rb#L50

This MR adds a note that users that look at this page and wish to auto link SAML users must use the SAML specific auto link setting instead.

Related issues

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