Fix two compiler warnings.

* CompoundAssignmentInstruction.cs (CS9336) — the C# 9 IntPtr / UIntPtr
  guard read `type.Kind is not TypeKind.NInt or TypeKind.NUInt`, which
  parses as `(is not NInt) or (is NUInt)` — true unless Kind == NInt.
  The intent (per the comment "but not nint or C# 11 IntPtr") is "Kind
  is neither NInt nor NUInt", which needs parentheses around the
  alternation: `is not (NInt or NUInt)`. Effect: when Kind == NUInt the
  branch no longer mistakenly applies the C# 9 IntPtr-only restrictions.

* IMethod.IsAsync (CS1574) — `<see cref="MethodImplAttributes.Async"/>`
  doesn't resolve on netstandard2.0 (the enum value was added later;
  this codebase declares its own SRMExtensions.MethodImplAsync = 0x2000
  for that reason). Rewrite the doc comment in prose to describe the
  flag without an unresolvable cref.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: siegfriedpammer

ICSharpCode.Decompiler/IL/Instructions/CompoundAssignmentInstruction.cs

@@ -215,7 +215,7 @@ internal static bool IsBinaryCompatibleWithType(BinaryNumericInstruction binary,
 						return false; // operator not supported on pointer types
 				}
 			}
-			else if ((type.IsKnownType(KnownTypeCode.IntPtr) || type.IsKnownType(KnownTypeCode.UIntPtr)) && type.Kind is not TypeKind.NInt or TypeKind.NUInt)
+			else if ((type.IsKnownType(KnownTypeCode.IntPtr) || type.IsKnownType(KnownTypeCode.UIntPtr)) && type.Kind is not (TypeKind.NInt or TypeKind.NUInt))
 			{
 				// If the LHS is C# 9 IntPtr (but not nint or C# 11 IntPtr):
 				// "target.intptr *= 2;" is compiler error, but

ICSharpCode.Decompiler/TypeSystem/IMethod.cs

@@ -113,7 +113,9 @@ public interface IMethod : IParameterizedMember
 		new IMethod Specialize(TypeParameterSubstitution substitution);
 
 		/// <summary>
-		/// Returns <see langword="true"/>, if the method is <see cref="MethodImplAttributes.Async"/>.
+		/// Returns <see langword="true"/> if the method has the runtime-async
+		/// <c>MethodImpl</c> flag set (the <c>0x2000</c> bit on
+		/// <see cref="System.Reflection.MethodImplAttributes"/>);
 		/// <see langword="false"/> otherwise.
 		/// </summary>
 		bool IsAsync { get; }