Add more doctest to `Monad` and `FlatMap` #2462

Job	Run time
Scalafix (ubuntu-latest, temurin@8)	3m 3s
Build and Test (ubuntu-latest, 2.12, temurin@8, rootJS)	12m 11s
Build and Test (ubuntu-latest, 2.12, temurin@8, rootJVM)	12m 51s
Build and Test (ubuntu-latest, 2.12, temurin@8, rootNative)	31m 49s
Build and Test (ubuntu-latest, 2.13, temurin@8, rootJS)	14m 50s
Build and Test (ubuntu-latest, 2.13, temurin@8, rootJVM)	14m 50s
Build and Test (ubuntu-latest, 2.13, temurin@8, rootNative)	29m 18s
Build and Test (ubuntu-latest, 2.13, temurin@17, rootJVM)	8m 12s
Build and Test (ubuntu-latest, 2.13, graalvm@17, rootJVM)	8m 55s
Build and Test (ubuntu-latest, 3, temurin@8, rootJS)	16m 28s
Build and Test (ubuntu-latest, 3, temurin@8, rootJVM)	14m 17s
Build and Test (ubuntu-latest, 3, temurin@8, rootNative)	29m 29s
Generate Site (ubuntu-latest, temurin@8)	6m 17s
Submit Dependencies	0s
Validate Steward Config (ubuntu-latest, temurin@11)	14s
Publish Artifacts	0s
	3h 22m 44s

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