Docs: sign-up/sign-in settings are not mentioned in relation to other auth methods

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We have extensive documentation on how to configure LDAP authentication/authorization in GitLab, but nowhere do we mention the effect of Sign-up or Sign-in restrictions in association with LDAP (or other external auth). We should somehow find a way to make it clear that users will still be able to register and/or sign in using local accounts no matter what external auth they configure, unless they disable these two items.

Reported by customer in internal link

~"docs-fix" customer