Clarify that first pipe character is optional in discriminated union syntax

[Copilot]

Fixes #7fabd101 (hypothetical issue reference based on the problem statement)

The syntax section of the discriminated unions documentation was missing an important detail: the first pipe character (|) is optional, particularly for single-case discriminated unions. This omission made it unclear that syntax like type X = Y of Z is valid.

Before:
The syntax section only showed the full syntax with pipe characters, leaving readers uncertain whether forms like type Address = Address of string were correct.

After:
Added explicit clarification that the first pipe character is optional, with a clear example:

type SingleCase = Case of string

This change ensures readers understand that both of these forms are valid:

• type Address = | Address of string (with pipe)
• type Address = Address of string (without pipe - commonly used for single-case DUs)

The addition is consistent with examples found throughout the F# documentation and aligns with the F# style guide recommendation to omit the leading pipe for single short unions.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Discriminated union syntax: Should mention that the 1st pipe character is optional</issue_title>
<issue_description>In the Syntax section at the beginning, it is important to state that the first 1st pipe character is optional.
Otherwise, it is not clear that "type X = Y of Z" is valid syntax for defining a single-case discriminated union.

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Document Details

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

• ID: 6020749e-d91e-8a1b-5366-25b212baa31d
• Version Independent ID: 0f28f508-dc0f-c272-383e-ec768759fe16
• Content: Discriminated Unions - F#
• Content Source: docs/fsharp/language-reference/discriminated-unions.md
• Product: dotnet-fsharp
• GitHub Login: @cartermp
• Microsoft Alias: phcart</issue_description>

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

Fixes #21971

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docs/fsharp/language-reference/discriminated-unions.md	docs/fsharp/language-reference/discriminated-unions

[BillWagner]

This is ready for final review.