Improve F# XML Documentation page with missing word and project file example

[Copilot]

Fixes #[issue_number]

This PR addresses two improvements to the F# XML Documentation page as requested in the issue:

Changes

1. Added missing word "approach"

Fixed the sentence structure when describing the second method of writing XML documentation comments. Changed "This second way" to "This second approach" for consistency with how the two methods are introduced earlier in the document.

Before:

This second way enables you to specify separate notes...

After:

This second approach enables you to specify separate notes...

2. Added project file example for --warnon:3390

Added a practical example showing how to enable XML documentation validation warnings using the project file approach. The documentation previously only mentioned the --warnon:3390 command-line option without showing the MSBuild/project file equivalent.

Added:

<WarnOn>3390</WarnOn>

This example is placed immediately after the paragraph explaining the --warnon:3390 option, making it easy for developers to find the project file approach. The format is consistent with other property examples in the document (such as GenerateDocumentationFile) and follows the pattern documented in the F# compiler-options.md file.

These minimal changes improve the clarity and completeness of the documentation, providing users with both command-line and project file configuration options.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve F# XML Docs page</issue_title>
<issue_description>In explaining the "second" approach, the word approach is missing.

Where --warnon is discussed, it would be nice to have an example of the project file approach to setting warnings.

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Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: 1a8fbc01-942f-ac15-e06b-2e50c45e8183
• Version Independent ID: 788235a1-2b43-1607-b24b-57c0552fd890
• Content: XML Documentation - F#
• Content Source: docs/fsharp/language-reference/xml-documentation.md
• Product: dotnet-fsharp
• GitHub Login: @cartermp
• Microsoft Alias: dotnetcontent</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #26636

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Internal previews

📄 File	🔗 Preview link
docs/fsharp/language-reference/xml-documentation.md	docs/fsharp/language-reference/xml-documentation

[BillWagner]

This is ready for final review, with one minor change.

[Copilot]

Pull Request Overview

This PR improves the F# XML Documentation page by addressing two specific documentation issues: adding a missing word for better sentence structure and providing a practical project file example for XML documentation warning configuration.

• Added the missing word "approach" to maintain consistency when describing the second method of writing XML documentation comments
• Added a project file example showing how to enable XML documentation validation warnings using <WarnOn>3390</WarnOn>