Replace outdated Array.Empty comments with correct reason

[Copilot]

Description

Addresses PR #123854 comment: replaces comments citing "Array.Empty() doesn't exist in all configurations" with the actual reason for using new T[0].

Changes

Updated pragma warning comments in 3 files:

• System.Diagnostics.DiagnosticSource/Activity.cs
• System.Threading.Tasks.Dataflow/Internal/ImmutableArray.cs
• System.Collections.Immutable/ImmutableArray_1.Minimal.cs

Before:

#pragma warning disable CA1825 // Array.Empty<T>() doesn't exist in all configurations

After:

#pragma warning disable CA1825 // avoid the extra generic instantiation for Array.Empty<T>()

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[Copilot]

Pull request overview

This PR addresses a comment from PR #123854 by updating outdated pragma warning comments that incorrectly stated "Array.Empty() doesn't exist in all configurations" with the actual reason: avoiding the extra generic instantiation for Array.Empty().

Changes:

• Updated pragma warning disable comments in three files to reflect the correct reason for using new T[0] instead of Array.Empty<T>()

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Threading.Tasks.Dataflow/src/Internal/ImmutableArray.cs	Updated CA1825 pragma comment from outdated "doesn't exist in all configurations" to "avoid the extra generic instantiation for Array.Empty()"
src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs	Updated CA1825 pragma comment with the same correction
src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray_1.Minimal.cs	Replaced two-line comment with single-line comment explaining the reason is to avoid extra generic instantiation