Update doc for IMethodSymbol.IsExtensionMethod

src/Compilers/Core/Portable/Symbols/IMethodSymbol.cs

@@ -37,8 +37,14 @@ public interface IMethodSymbol : ISymbol
         bool IsGenericMethod { get; }
 
         /// <summary>
-        /// Returns true if this method is an extension method. 
+        /// Returns true if this method is a "classic" extension method (using the <see langword="this"/>
+        /// modifier in C# or <see cref="System.Runtime.CompilerServices.ExtensionAttribute"/> in VB).
         /// </summary>
+        /// <remarks>
+        /// Returns false for methods in <c>extension()</c> blocks.
+        /// To check if a method is a "new" extension method (a member of an <c>extension()</c> block),
+        /// check <see cref="ITypeSymbol.IsExtension"/> on the method's <see cref="ISymbol.ContainingType"/>.
+        /// </remarks>
         bool IsExtensionMethod { get; }
 
         /// <summary>

src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Extensions/Symbols/ISymbolExtensions.cs

@@ -271,6 +271,7 @@ public static bool IsReducedExtension([NotNullWhen(true)] this ISymbol? symbol)
     public static bool IsEnumMember([NotNullWhen(true)] this ISymbol? symbol)
         => symbol is { Kind: SymbolKind.Field, ContainingType.TypeKind: TypeKind.Enum };
 
+    /// <inheritdoc cref="IMethodSymbol.IsExtensionMethod"/>
     public static bool IsExtensionMethod(this ISymbol symbol)
         => symbol is IMethodSymbol { IsExtensionMethod: true };