It's spread throughout the 'Playing the Game' chapter. The Grapple section explicitly offers a choice ("or" not and) between supplemental actions. What you can do on YS199, the example on YS212, the description of free and supplemental actions on YS213, and the definition on YS401 all refer to supplemental actions in the singular while referring to free actions in the plural. Meanwhile free actions explicitly allow you to break the one action per turn rule but, if you do enough of them, may become a (singular) supplemental action. Overflow actions are discussed on YS214.