Improve UI text of admin settings page - General -> Sourcegraph

What does this MR do and why?

Improves UI text of the admin area's Settings > General > Sourcegraph page.

Details of change:

• Changed More information link.'s text to Learn more.. I couldn't think of a suitable question.
• Change description to state the effect when checked.
• Sourcegraph URL - removed e.g. from placeholder text.

All changes are intended to improve compliance with the GitLab design system.

Part of #350108 (closed)

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