Fix a docs string

@sushant-hiray, I think you added this description in 1d4994b0ba4d1ab12487e136a534510c5f96456e, let me know if you agree.

I looked at `sin` and `cos` in `functions.h`. Your comment is actually correct --- these functions do the canonicalization first, i.e. if you call `sin(0)` it returns 0 immediately, only creates an instance of `Sin` if it cannot be simplified. 

For FunctionSymbol, we just return FunctionSymbol, as there is not much that we can do.

Maybe we should leave your original comment.