Document ToString() conversion for F# interpolated strings

[Copilot]

Summary

This PR addresses issue #45234 by clarifying how expression-to-string conversion works in F# interpolated strings.

Problem

The documentation for F# interpolated strings explained typed interpolated strings (with format specifiers like %s{expr}) but did not specify what conversion mechanism is used for non-typed interpolated strings (simple {expr} syntax). Users were left uncertain about how their expressions would be converted to strings.

Solution

Added a clarification in the Remarks section that explicitly documents:

• Non-typed interpolated strings use the ToString() method to convert expressions to strings
• When an expression evaluates to null, an empty string is used instead
• Typed interpolated strings with format specifiers follow the rules defined for that specific format specifier

This aligns with the F# compiler implementation as confirmed by @T-Gro's reference to the source code.

Example

// Non-typed: uses ToString()
let age = 30
printfn $"Age: {age}"  // Calls age.ToString()

// Typed: follows format specifier rules
printfn $"Age: %d{age}"  // Uses the %d format specifier conversion

Also added ai-usage: ai-assisted frontmatter per .NET documentation guidelines.

Original prompt

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This section details on the original issue you should resolve

<issue_title>For Interpolated Strings, we should indicate what kind of conversion is used.</issue_title>
<issue_description>
While for "Typed Interpolated Strings" the conversion of the expression to a string is precisely defined, there is no such indication for non-typed conversion. I assume the string operator is used. Briefly mentioning this in the Remarks section would be helpful.

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Document Details

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

• ID: 178dbf92-8d00-887f-f55c-44ae4ce0fc26
• Version Independent ID: fa136057-4782-c009-437b-f08bdd267f6a
• Content: Interpolated strings - F#
• Content Source: docs/fsharp/language-reference/interpolated-strings.md
• Product: dotnet-fsharp
• GitHub Login: @cartermp
• Microsoft Alias: dotnetcontent</issue_description>

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

@BillWagner ping @dsyme for your thoughts here. @BillWagner @T-Gro

When I read the spec, it looks like the conversion does use the string operation, unless the destination is an IFormattable. If so, I'll get a fix in place.</comment_new>
<comment_new>@T-Gro
For interpolated strings, I believe it uses ToString() (unless null).
https://github.com/dotnet/fsharp/blob/bd346da564d516d60bec00c509315ac1903e64e4/src/FSharp.Core/printf.fs#L907-L933

Fixes #27048

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Internal previews

📄 File	🔗 Preview link
docs/fsharp/language-reference/interpolated-strings.md	Interpolated strings

[BillWagner]

This is ready for final review.

[Copilot]

Pull Request Overview

This PR addresses issue #45234 by clarifying the string conversion mechanism used in F# interpolated strings. The documentation previously explained typed interpolated strings but left users uncertain about how expressions are converted to strings in non-typed interpolated strings.

• Added explicit documentation for ToString() conversion in non-typed interpolated strings
• Clarified null handling behavior (converts to empty string)
• Distinguished between non-typed and typed interpolated string conversion rules
• Added required AI usage frontmatter per .NET documentation guidelines