Document ToString() conversion for F# interpolated strings (#49134)

* Initial plan

* Document ToString() conversion for F# interpolated strings

Co-authored-by: BillWagner <[email protected]>

* Add ai-usage frontmatter to indicate AI assistance

Co-authored-by: BillWagner <[email protected]>

* Update docs/fsharp/language-reference/interpolated-strings.md

Co-authored-by: Copilot <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: BillWagner <[email protected]>
Co-authored-by: Bill Wagner <[email protected]>
Co-authored-by: Copilot <[email protected]>

Author: 3 people

docs/fsharp/language-reference/interpolated-strings.md

@@ -2,6 +2,7 @@
 title: Interpolated strings
 description: Learn about interpolated strings, a special form of string that allows you to embed F# expressions directly inside them.
 ms.date: 10/01/2025
+ai-usage: ai-assisted
 ---
 
 # Interpolated strings
@@ -33,6 +34,9 @@ printfn $"I think {3.0 + 0.14} is close to {System.Math.PI}!"
 
 The contents in between each `{}` brace pair can be any F# expression.
 
+For non-typed interpolated strings (without format specifiers), the expression is converted to a string using the `ToString()` method. If the expression evaluates to `null`, an empty string is used.
+
+For typed interpolated strings with format specifiers (such as `%s{expr}` or `%d{expr}`), the conversion follows the rules defined for that specific format specifier.
 To escape a `{}` brace pair, write two of them like so:
 
 ```fsharp