Add docs on ExtendedLayout (#51762)

* Add docs on ExtendedLayout

Fixes #51746

Add description of the `ExtendedLayoutAttribute` in the language attributes section.

* Update docs/csharp/language-reference/attributes/general.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Apply suggestion from @BillWagner

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: BillWagner

docs/csharp/language-reference/attributes/general.md

@@ -1,6 +1,6 @@
 ---
 title: "Attributes interpreted by the compiler: Miscellaneous"
-ms.date: 01/14/2026
+ms.date: 02/17/2026
 description: "Learn about attributes that affect code generated by the compiler: the Conditional, Obsolete, AttributeUsage, ModuleInitializer, and SkipLocalsInit attributes."
 ---
 # Miscellaneous attributes interpreted by the C# compiler
@@ -26,6 +26,7 @@ You can apply several attributes to elements in your code to add semantic meanin
 - [`InlineArray`](#inlinearray-attribute): Specify that a `struct` type is an *inline array*.
 - [`IUnknownConstant`](#iunknownconstant-and-idispatchconstant-attributes): Specifies how a missing argument should be supplied for a default parameter.
 - [`IDispatchConstant`](#iunknownconstant-and-idispatchconstant-attributes): Specifies how a missing argument should be supplied for a default parameter.
+- [`ExtendedLayoutAttribute`](#extendedlayout-attribute): Instructs the compiler to set the `ExtendedLayout` flag in the type's metadata.
 
 The compiler uses those semantic meanings to alter its output and report possible mistakes by developers using your code.
 
@@ -273,6 +274,16 @@ The <xref:System.Runtime.CompilerServices.CollectionBuilderAttribute?displayProp
 
 The <xref:System.Runtime.CompilerServices.InlineArrayAttribute?displayProperty=nameWithType> attribute marks a type as an [inline array](../builtin-types/struct.md#inline-arrays). You can learn more about this feature in the article on `structs`, in the section on [inline arrays](../builtin-types/struct.md#inline-arrays).
 
+> [!IMPORTANT]
+> This attribute can't be combined with the [`ExtendedLayout`](#extendedlayout-attribute) attribute.
+
+## ExtendedLayout attribute
+
+The <xref:System.Runtime.InteropServices.ExtendedLayoutAttribute?displayProperty=nameWithType> instructs the compiler to set the <xref:System.Reflection.TypeAttributes.ExtendedLayout?displayProperty=nameWithType> flag in the type's <xref:System.Type.Attributes?displayProperty=nameWithType> flags.
+
+> [!IMPORTANT]
+> This attribute can't be combined with the [`StructLayoutAttribute`](pseudo-attributes.md) or the [`InlineArray`](#inlinearray-attribute) attribute.
+
 ## IUnknownConstant and IDispatchConstant attributes
 
 The <xref:System.Runtime.CompilerServices.IUnknownConstantAttribute?displayProperty=nameWithType> and <xref:System.Runtime.CompilerServices.IDispatchConstantAttribute?displayProperty=nameWithType> attributes can be added to default parameters to indicate that a missing argument should be supplied as `new UnknownWrapper(null)` or `new DispatchWrapper(null)`.

docs/csharp/language-reference/attributes/pseudo-attributes.md

@@ -1,6 +1,6 @@
 ---
 title: "Attributes interpreted by the compiler: Pseudo-attributes"
-ms.date: 01/14/2026
+ms.date: 02/17/2026
 description: "Learn about attributes you can add to code that are written to IL as modifiers. These custom attributes aren't emitted as attributes in the compiled output."
 ---
 # Custom attributes that generate flags or options in the Intermediate Language (IL) output
@@ -23,6 +23,9 @@ Add these attributes to your code for the compiler to emit a specified Intermedi
 | <xref:System.Runtime.InteropServices.StructLayoutAttribute?displayProperty=fullName> | `auto`, `sequential`, or `explicit` | Layout options can be set using the parameters. |
 | <xref:System.Runtime.CompilerServices.IndexerNameAttribute?displayProperty=fullName> |                 | Add this attribute to an indexer to set a different method name. By default, indexers are compiled to a property named `Item`. You can specify a different name using this attribute. |
 
+> [!IMPORTANT]
+> The [`StructLayoutAttribute`] can't be combined with the [`ExtendedLayout`](general.md#extendedlayout-attribute) attribute.
+
 Some of these custom attributes are applied by using other C# syntax rather than adding the attribute to your source code.
 
 | Attribute                                                                                        | Comments |