webdriver: Improve documentations for actions.rs and update TODOs (servo#39391)

Add specification quote for each steps, remove some irrelevant TODO, and
move function position closer to related function.

Testing: No behaviour change

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Signed-off-by: PotatoCP <[email protected]>
Signed-off-by: Euclid Ye <[email protected]>
Co-authored-by: Euclid Ye <[email protected]>

Author: PotatoCP