Update ServiceNow credentials doc to reflect changes on PR 17926

[Heknon]

n8n-io/n8n#17926

Until now, ServiceNow credentials in n8n accepted only a sub-domain, which excluded
users whose instances sit behind a reverse proxy or run on a completely custom host.
This PR adds an optional “Use custom host?” toggle plus Custom Host field to both
Basic-Auth and OAuth2 credentials, enabling any ServiceNow URL.

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