add args check and comment for exp,polynomy decay

[betterpig]

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Describe

icafe issue[DLTP-38875]

add args check and comment for exponential,polynomial decay. Theoretically, these two kinds of function should restrict exponent's ranges to get learning rate actually decay.

[paddle-bot-old]

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[zhwesky2010]

最好用数学的形式，直接写成区间范围

[betterpig]

最好用数学的形式，直接写成区间范围

对于有界区间，我改成用数学形式(a, b)表示。对于单测无界区间，考虑到无穷符号∞在编译显示时可能出问题，还是用文字形式表述。