Implement "closed cases only" message on MUR search page

[dorothyyeager]

Summary

What we are after:
We need to let users of the MUR search know that it doesn't include open and ongoing cases, but only closed ones.

Related issues

List any relevant related issue(s)

• Add MUR filter to datatable #2969
• Craft message for MUR search to indicate it only includes closed cases #3049

Completion criteria

• Change label on MUR Search page https://www.fec.gov/data/legal/search/enforcement/?case_no= to read "Closed Matters Under Review" instead of "Matters under review."
• Add "closed" to the first sentence in the info message at the top that explains what's in our search, so that it reads:

The MUR search feature includes all closed cases dating back to . . .

• Include content team member in the PR review for proofreading.

[patphongs]

@dorothyyeager Would like to pair on how these changes are made