Improve neatness of sequence diagrams

[nus-pe-bot]

Note from the teaching team: This bug was reported during the Part II (Evaluating Documents) stage of the PE. You may reject this bug if it is not related to the quality of documentation.

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Trivial methods can be excluded from the sequence diagrams to improve readability.

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_{[original: nus-cs2113-AY2223S1/pe-interim#67] [original labels: severity.VeryLow type.DocumentationBug]}

[gaoyunfan]

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