DOC: merge_asof accepts a vector for 'left_on' parameter (and this is great! :))
#### Documentation problem

[pandas.merge_asof()](https://pandas.pydata.org/docs/reference/api/pandas.merge_asof.html) states regarding `left_on`parameter:

_left_on : label
Field name to join on in left DataFrame._

#### Suggested fix for documentation

I tried to see if like `merge_ordered` a vector the size of the left DataFrame would not be accepted, and it is. I would suggest to complete the parameter description similarly to what is stated in [`merge_ordered()`](https://pandas.pydata.org/pandas-docs/version/1.2.4/reference/api/pandas.merge_ordered.html)

_left_on : label or list, or array-like
Field names to join on in left DataFrame. Can be a vector or list of vectors of the length of the DataFrame to use a particular vector as the join key instead of columns._
