Document ICopyable and add id checks to CopyFrom implementation

Author: shana

src/GitHub.App/Models/PullRequestModel.cs

@@ -19,6 +19,8 @@ public PullRequestModel(int number, string title, IAccount author, DateTimeOffse
 
         public void CopyFrom(IPullRequestModel other)
         {
+            if (Number != other.Number)
+                throw new ArgumentException("Instance to copy from must match", nameof(other));
             Title = other.Title;
             UpdatedAt = other.UpdatedAt;
             CommentCount = other.CommentCount;

src/GitHub.Exports/Collections/ICopyable.cs

@@ -1,5 +1,11 @@
 namespace GitHub.Collections
 {
+    /// <summary>
+    /// Allows a type instance to copy data from another
+    /// instance. Similar to a Clone action, but without
+    /// creating a new instance.
+    /// </summary>
+    /// <typeparam name="T">Type that implements CopyFrom</typeparam>
     public interface ICopyable<in T>
     {
         void CopyFrom(T other);