PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

The Signal Forms validation guide documents validation helpers such as applyEach(), but it does not explain how to apply validation conditionally for union or variant-based forms using applyWhenValue().

Issue Number: #66906

What is the new behavior?

Adds a new section to the Signal Forms validation guide showing how to use applyWhenValue() with discriminated unions.

The example demonstrates how different validation rules can be applied depending on the active variant.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Inspired by the discussion and examples linked in the issue and StackOverflow references.