updated docstrings for onset backtracking
updated docstrings for onset backtracking

ruohoruotsi/librosa,stefan-balke/librosa,r9y9/librosa,carlthome/librosa,r9y9/librosa,r9y9/librosa,stefan-balke/librosa,ruohoruotsi/librosa,ruohoruotsi/librosa,carlthome/librosa,stefan-balke/librosa,carlthome/librosa