Add puml files for tutorial class diagrams

Two class diagrams in 'Add Remark Command' tutorial don't
have corresponding .puml files. Furthermore, the diagrams
have some errors.

Let's correct the errors, and also add the missing .puml files.

Also, let's rename the files to more suitable names.

docs/diagrams/add-remark/ParserClass.puml

@@ -0,0 +1,14 @@
+@startuml
+hide circle
+skinparam classAttributeIconSize 0
+
+Class "<<interface>>\nParser" as Parser
+Class RemarkCommandParser {
+  +parse(): RemarkCommand
+}
+Class ParserException
+
+RemarkCommandParser .up.|> Parser
+Parser .right.> ParserException: throws >
+RemarkCommandParser .right.> ParserException: throws >
+@enduml

docs/diagrams/add-remark/RemarkClass.puml

@@ -0,0 +1,19 @@
+@startuml
+hide circle
+skinparam classAttributeIconSize 0
+
+Class "{abstract}\nCommand" as Command {
+  +execute(Model): CommandResult
+}
+Class RemarkCommand {
+  +COMMAND_WORD: String
+  +MESSAGE_USAGE: String
+  +MESSAGE_NOT_IMPLEMENTED_YET: String
+  +execute(Model): CommandResult
+}
+Class CommandException
+
+RemarkCommand -up-|> Command
+Command ..> CommandException: throws >
+RemarkCommand .right.> CommandException: throws >
+@enduml

docs/tutorials/AddRemark.md

@@ -57,7 +57,7 @@ Run `Main#main` and try out your new `RemarkCommand`. If everything went well, y
 
 While we have successfully printed a message to `ResultDisplay`, the command does not do what it is supposed to do. Let’s change the command to throw a `CommandException` to accurately reflect that our command is still a work in progress.
 
-![The relationship between RemarkCommand and Command](../images/add-remark/CommandInterface.png)
+![The relationship between RemarkCommand and Command](../images/add-remark/RemarkCommandClass.png)
 
 Following the convention in other commands, we add relevant messages as constants and use them.
 
@@ -142,7 +142,7 @@ Now let’s move on to writing a parser that will extract the index and remark f
 
 Create a `RemarkCommandParser` class in the `seedu.address.logic.parser` package. The class must extend the `Parser` interface.
 
-![The relationship between Parser and RemarkCommandParser](../images/add-remark/ParserInterface.png)
+![The relationship between Parser and RemarkCommandParser](../images/add-remark/RemarkCommandParserClass.png)
 
 Thankfully, `ArgumentTokenizer#tokenize()` makes it trivial to parse user input. Let’s take a look at the JavaDoc provided for the function to understand what it does.