Add introduction to arrow syntax in for...in expression documentation

[Copilot]

Summary

Fixes the issue where the arrow (->) operator syntax appeared in a code example without prior introduction or explanation.

Changes

Added a brief introduction to the arrow syntax before the first code example that uses it in the loops-for-in-expression.md document. The addition:

• Explains that -> is a shorthand for do yield in sequence expressions and computation expressions
• Clarifies that it allows you to specify an expression whose value becomes part of the sequence
• Provides links to the Sequences and Computation Expressions documentation for readers seeking more detailed information

Example

The code snippet that was appearing without context:

let seq1 = seq { for i in 1 .. 10 -> (i, i*i) }

Now has an introduction explaining the -> operator before readers encounter it in the example.

Closes #[issue_number]

Original prompt

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This section details on the original issue you should resolve

<issue_title>Missing introduction of arrow syntax </issue_title>
<issue_description>Using the error approach appears without introduction in a sample. Suggest this appear at the top with the initial syntax introduction.

let seq1 = seq { for i in 1 .. 10 -> (i, i*i) }

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Document Details

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

• ID: 4f0880ed-6840-b0e9-ee73-f56a60b7a3e5
• Version Independent ID: a93b42e0-1fa3-ccdc-315c-268a86669383
• Content: Loops: for...in Expression - F#
• Content Source: docs/fsharp/language-reference/loops-for-in-expression.md
• Product: dotnet-fsharp
• GitHub Login: @cartermp
• Microsoft Alias: dotnetcontent</issue_description>

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

@BillWagner In addition to a brief introduction to the syntax, the fix should provide links to more details on this syntax.

Fixes #26356

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Internal previews

📄 File	🔗 Preview link
docs/fsharp/language-reference/loops-for-in-expression.md	docs/fsharp/language-reference/loops-for-in-expression

[BillWagner]

This is ready for final review.

[Copilot]

Pull Request Overview

This PR adds documentation to clarify the arrow (->) operator syntax in F# sequence expressions that was previously appearing without introduction in a code example.

• Adds an explanation of the -> operator as shorthand for do yield in sequence and computation expressions
• Provides context links to more detailed documentation on Sequences and Computation Expressions
• Addresses the issue where syntax appeared in examples without prior explanation