Streamline docs for Attachment

+ This commit also adds further explanation for why Embeds and Attachments are read-only collections

src/Discord.Net.Core/Entities/Messages/IAttachment.cs

@@ -1,7 +1,7 @@
 namespace Discord
 {
     /// <summary>
-    ///     Represents a Discord attachment.
+    ///     Represents a message attachment found in a <see cref="IUserMessage"/>.
     /// </summary>
     public interface IAttachment
     {

src/Discord.Net.Core/Entities/Messages/IMessage.cs

@@ -64,6 +64,11 @@ namespace Discord
         /// <summary>
         ///     Gets all attachments included in this message.
         /// </summary>
+        /// <remarks>
+        ///     This property gets a read-only collection of attachments associated with this message. Depending on the
+        ///     user's end-client, a sent message may contain one or more attachments. For example, mobile users may
+        ///     attach more than one file in their message, while the desktop client only allows for one.
+        /// </remarks>
         /// <returns>
         ///     A read-only collection of attachments.
         /// </returns>
@@ -71,6 +76,11 @@ namespace Discord
         /// <summary>
         ///     Gets all embeds included in this message.
         /// </summary>
+        /// <remarks>
+        /// </remarks>
+        ///     This property gets a read-only collection of embeds associated with this message. Depending on the
+        ///     message, a sent message may contain one or more embeds. This is usually true when multiple link previews
+        ///     are generated; however, only one <see cref="EmbedType.Rich"/> <see cref="Embed"/> can be featured.
         /// <returns>
         ///     A read-only collection of embed objects.
         /// </returns>

src/Discord.Net.Rest/Entities/Messages/Attachment.cs

@@ -3,9 +3,7 @@ using Model = Discord.API.Attachment;
 
 namespace Discord
 {
-    /// <summary>
-    ///     An attachment file seen in a <see cref="IUserMessage"/>.
-    /// </summary>
+    /// <inheritdoc/>
     [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
     public class Attachment : IAttachment
     {