Add missing xml documentation comments for few public IOperation APIs

Fixes #22692

src/Compilers/Core/Portable/Operations/ISymbolInitializer.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-using System.Collections.Immutable;
-
 namespace Microsoft.CodeAnalysis.Semantics
 {
     /// <summary>
-    /// Represents an initializer for a field, property, or parameter.
+    /// Represents an initializer for a field, property, parameter or a local variable declaration.
     /// </summary>
     /// <remarks>
     /// This interface is reserved for implementation by its associated APIs. We reserve the right to
@@ -13,6 +11,9 @@ namespace Microsoft.CodeAnalysis.Semantics
     /// </remarks>
     public interface ISymbolInitializer : IOperation
     {
+        /// <summary>
+        /// Underlying initializer value.
+        /// </summary>
         IOperation Value { get; }
     }
 }

src/Compilers/Core/Portable/Operations/OperationExtensions.cs

@@ -6,7 +6,6 @@
 using System.Linq;
 using System.Threading;
 using Microsoft.CodeAnalysis.PooledObjects;
-using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.Semantics
 {
@@ -38,11 +37,19 @@ internal static bool HasErrors(this IOperation operation, Compilation compilatio
             return model.GetDiagnostics(operation.Syntax.Span, cancellationToken).Any(d => d.DefaultSeverity == DiagnosticSeverity.Error);
         }
 
+        /// <summary>
+        /// Returns all the descendant operations of the given <paramref name="operation"/> in evaluation order.
+        /// </summary>
+        /// <param name="operation">Operation whose descendants are to be fetched.</param>
         public static IEnumerable<IOperation> Descendants(this IOperation operation)
         {
             return Descendants(operation, includeSelf: false);
         }
 
+        /// <summary>
+        /// Returns all the descendant operations of the given <paramref name="operation"/> including the given <paramref name="operation"/> in evaluation order.
+        /// </summary>
+        /// <param name="operation">Operation whose descendants are to be fetched.</param>
         public static IEnumerable<IOperation> DescendantsAndSelf(this IOperation operation)
         {
             return Descendants(operation, includeSelf: true);
@@ -88,6 +95,10 @@ private static IEnumerable<IOperation> Descendants(IOperation operation, bool in
             stack.Free();
         }
 
+        /// <summary>
+        /// Gets all the declared local variables in the given <paramref name="declarationStatement"/>.
+        /// </summary>
+        /// <param name="declarationStatement">Variable declaration statement</param>
         public static ImmutableArray<ILocalSymbol> GetDeclaredVariables(this IVariableDeclarationStatement declarationStatement)
         {
             if (declarationStatement == null)

src/Compilers/Core/Portable/Operations/OperationKind.cs

@@ -9,6 +9,9 @@ namespace Microsoft.CodeAnalysis
     /// </summary>
     public enum OperationKind
     {
+        /// <summary>
+        /// Indicates an <see cref="IOperation"/> for an not yet implemented semantic construct.
+        /// </summary>
         None = 0x0,
 
         // Statements