Updates wording for Restricted Contexts requirements

[alanjclark]

Description

Clarifies that Restricted Contexts is available on GH App pipelines on GH Oauth connected orgs.

Reasons

Wording was implying that this was only available on GH OAuth

Content Checklist

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[alanjclark]

Updated with review recommendations, and also added exception for custom webhooks as per Slack conversation.

Sebastian is on PTO