Fixes #59398

PR Checklist

• 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
• Docs have been added / updated (not required for error message improvement)

PR Type

• Bugfix

What is the current behavior?

The diagnostic error NG8011 (controlFlowPreventingContentProjection) suggests wrapping content in an <ng-container/> but does not mention ngProjectAs, which is the key attribute needed to make the workaround actually work for named slots.

Users following the existing suggestion have no way to know they need ngProjectAs="[selector]" to target a specific slot, leading to confusion (as reported in the issue).

Additionally, the message does not clarify when to use the wrapping approach vs. splitting into multiple blocks, which are solutions for different scenarios (single-slot vs. multi-slot targeting).

What is the new behavior?

Option 1 of the diagnostic now explicitly shows <ng-container ngProjectAs="[selector]"> with the actual slot selector interpolated, and explains this is the right approach when all nodes target the same slot.

Option 2 now clarifies it is needed when nodes target different slots.

Example

Before:

1. Wrap the content of the @if block in an <ng-container/> that matches the "[foo]" selector.
2. Split the content of the @if block across multiple @if blocks such that each one only has a single projectable node at its root.

After:

1. Wrap the content of the @if block in an <ng-container ngProjectAs="[foo]"> to explicitly target the "[foo]" slot. Use this when all nodes in the block should be projected into the same slot.
2. Split the content of the @if block across multiple @if blocks such that each one only has a single projectable node at its root. Use this when nodes in the block target different slots.