[Improvement 3] Adding Javadoc everywhere

[Lei-Tin]

Currently, none of our pull request had included Javadocs. I believe this is also one of the requirements of the project - Adding comments and Javadoc everywhere so that other people can understand our code easily.

Features / Pull Requests that needs to add Javadocs

• [Documentation] Javadoc for UserJoinUseCase @xgao28 #55
• [Documentation] Javadoc for BillUpdateUseCase @ScottCTD #53
• [Documentation] Javadoc for InsertEntryUseCase @Yunshan-522 #56
• [Documentation] Javadoc for DeleteEntryUseCase @Ellen141319 #57
• [Documentation] Javadoc for AlterEntryUseCase @Yuhan-Ut #58
• [Documentation] Javadoc for MySQLDatabaseGateway @Lei-Tin #54

Aside from adding Javadoc, you can also go into the Wiki of this repo to include more documentation for your methods.