Docs: fix popover option types

[Kevinjohn]

Description

Fixes the documented types for three popover options.

Motivation & Context

The popover options table had drifted from the JavaScript DefaultType definitions shared with Tooltip and extended by Popover:

Expected: documented option types match the accepted runtime config types.
Actual: fallbackPlacements included string, offset included number and omitted array, and content omitted null.

This updates the Popover docs table so those rows match the source types.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-42707--twbs-bootstrap.netlify.app/

Related issues

None.

Checks

• git diff --check
• npm exec -- prettier --config site/.prettierrc.json -c site/src/content/docs/components/popovers.mdx
• npm run docs-build