insights: docs updates regarding scale/limitations

[chwarwick]

Document new insights settings for controlling backfills
Update docs regarding limitations and scale.

Test plan

n/a docs only

[github-actions]

❌ Problem: the label i-acknowledge-this-goes-into-the-release is absent.
👉 What to do: we're in the next Sourcegraph release code freeze period. If you are 100% sure your changes should get released or provide no risk to the release, add the label your PR with i-acknowledge-this-goes-into-the-release.
❓ When does the freeze end? The code freeze is active until 2023-02-22 at 23:59 UTC.

[sourcegraph-bot]

Codenotify: Notifying subscribers in CODENOTIFY files for diff 757d369...470bcd9.

Notify	File(s)
@sourcegraph/delivery	doc/code_insights/explanations/administration_and_security_of_code_insights.md