Add an introduction of the "before/after switch" to the "learning session"

[deeper747]

Is your feature request related to a problem? Please describe.
：有沒有擴建，比較難，因為我常過度聚焦在看準心處，沒有看整體，就回答了，於是下一張問有無擴建時，只能瞎猜。
：圈起來的地方可以切換before and after~
：原來如此！教學沒有提示，還以為只是在告知before/after，不知可以切換。感謝。
：deepr使用者回饋～我一開始玩也沒注意到before and after

Describe the solution you'd like
Add an introduction of the switch to the "learning session."

Describe alternatives you've considered
Redesign the before/after button so that users could notice it easily.

Additional context
I've also remind a friend to use the button when he reach the third question and couldn't make a decision.