Fix exception summary to comply with official Microsoft Docs style

* References
https://docs.microsoft.com/en-us/dotnet/api/system.argumentnullexception?view=netframework-4.7.2
https://docs.microsoft.com/en-us/dotnet/api/system.platformnotsupportedexception?view=netframework-4.7.2
https://docs.microsoft.com/en-us/dotnet/api/system.badimageformatexception?view=netframework-4.7.2

src/Discord.Net.Commands/CommandException.cs

@@ -3,7 +3,7 @@ using System;
 namespace Discord.Commands
 {
     /// <summary>
-    ///     Describes an exception that occurred during a command execution.
+    ///     The exception that is thrown if another exception occurs during a command execution.
     /// </summary>
     public class CommandException : Exception
     {

src/Discord.Net.Core/Net/HttpException.cs

@@ -4,7 +4,7 @@ using System.Net;
 namespace Discord.Net
 {
     /// <summary>
-    ///     Describes an exception that occurred during the processing of Discord HTTP requests.
+    ///     The exception that is thrown if an error occurs while processing an Discord HTTP request.
     /// </summary>
     public class HttpException : Exception
     {

src/Discord.Net.Core/Net/WebSocketClosedException.cs

@@ -2,7 +2,7 @@ using System;
 namespace Discord.Net
 {
     /// <summary>
-    ///     Describes an exception that causes the WebSocket to close during a session.
+    ///     The exception that is thrown when the WebSocket session is closed by Discord.
     /// </summary>
     public class WebSocketClosedException : Exception
     {