Because it is absent from GenTraversableOnce (yet not from TraversableOnce), reduceLeft does not seem to have a documentation string.
Yet, it is referred to directly in the documentation string of reduceLeftOption. To quote:
Optionally applies a binary operator to all elements of this collection or iterator, going left to right.
return an option value containing the result of reduceLeft(op) is this collection or iterator is nonempty, None otherwise.