Fix attribute names for autocomplete and SSH key rate limits in settings API docs

What does this MR do?

Corrects four attribute names in doc/api/settings.md that do not match what PUT /application/settings accepts and GET /application/settings returns:

Documented	Actual
autocomplete_users	autocomplete_users_limit
autocomplete_users_unauthenticated	autocomplete_users_unauthenticated_limit
users_api_limit_keys	users_api_limit_ssh_keys
users_api_limit_key	users_api_limit_ssh_key

lib/api/settings.rb and app/models/application_setting.rb have used the "Actual" names since these settings were introduced in GitLab 17.10. Verified on GitLab 19.4.1-ee and 19.5.0-pre.

Related issues

Closes #631701

Author's checklist

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