docs: add xml comments for expr tree operators

Agent-Logs-Url: https://github.com/dadhi/FastExpressionCompiler/sessions/306086af-4c82-4f97-9286-a3fe568d3f9a

Co-authored-by: dadhi <39516+dadhi@users.noreply.github.com>

Author: Copilot

src/FastExpressionCompiler.LightExpression/FlatExpression.cs

@@ -738,9 +738,11 @@ public override bool Equals(object obj) =>
     public override int GetHashCode() =>
         new StructuralComparer().Hash(ref this);
 
+    /// <summary>Determines whether two flat expression trees are structurally equal.</summary>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static bool operator ==(ExprTree left, ExprTree right) => left.Equals(right);
 
+    /// <summary>Determines whether two flat expression trees are not structurally equal.</summary>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static bool operator !=(ExprTree left, ExprTree right) => !left.Equals(right);