Clarify FixedAddressValueTypeAttribute applicability and static field storage wording

[Copilot]

This PR updates FixedAddressValueTypeAttribute documentation to align with runtime feedback on attribute applicability. It also replaces an implementation-detail statement with wording that better reflects actual static field storage behavior.

• Applicability clarification

  • Explicitly documents that the attribute does not apply to primitive value type and enum fields.

• Remarks wording update

  • Replaces “Static value type fields are created as boxed objects.” with “The storage for static fields may be allocated on GC heap.”

• Updated doc text (excerpt)

  Use the <xref:System.Runtime.CompilerServices.FixedAddressValueTypeAttribute> attribute to mark static value types as pinned upon creation. This attribute does not apply to primitive value type and enum fields.
  
  The storage for static fields may be allocated on GC heap. This means that their address can change as garbage collection is performed.

[Copilot]

Pull request overview

This PR updates the ECMA XML documentation for System.Runtime.CompilerServices.FixedAddressValueTypeAttribute to better reflect the attribute’s actual applicability and to avoid overstating an implementation detail about static field storage.

Changes:

• Clarifies that FixedAddressValueTypeAttribute applies to static value type fields, and that it doesn’t apply to primitive value type fields or enum fields.
• Rewords the remarks about static field storage to reflect that storage can be on the GC heap, which allows addresses to change during garbage collection.