Fix 'Accept pipeline input' properties in documentation of Invoke-Formatter

[bergmeister]

PR Summary

PR #1763 made Invoke-Formatter accept pipeline input but tests didn't catch that cmdlet docs weren't updated. This fixes the docs and adds tests for pipeline input. Since that pull request hasn't been released yet, no need to update Microsoft docs.

PR Checklist

• [x] PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• [x] Summarized changes
• [x] Change is not breaking
• [x] Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• [x] Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• [x] This PR is ready to merge and is not Work in Progress.
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