Add missing Severity entry for Invoke-ScriptAnalyzer docs

[liamjpeters]

PR Summary

The -Severity parameter of the Invoke-ScriptAnalyzer command has 4 allowed values.

PSScriptAnalyzer/Engine/Commands/InvokeScriptAnalyzerCommand.cs

Lines 148 to 159 in 4b0117c

	/// <summary>
	/// IncludeRule: Array of the severity types to be enabled.
	/// </summary>
	[ValidateSet("Warning", "Error", "Information", "ParseError", IgnoreCase = true)]
	[Parameter(Mandatory = false)]
	[SuppressMessage("Microsoft.Performance", "CA1819:PropertiesShouldNotReturnArrays")]
	public string[] Severity
	{
	get { return severity; }
	set { severity = value; }
	}
	private string[] severity;

The docs are missing ParseError as an allowed value.

Fixes #2192

Does not conflict with #2190

PR Checklist

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

[Copilot]

Pull request overview

Updates the Invoke-ScriptAnalyzer cmdlet documentation to match the actual allowed -Severity values implemented in InvokeScriptAnalyzerCommand (including ParseError).

Changes:

• Add ParseError to the documented valid values for -Severity.
• Normalize the valid-values list formatting (remove trailing punctuation from Information).

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