ENH: Allow passing multiple axes as df.boxplot's `ax` argument
As discussed in #6991. At the moment with `ax` accepting a single axis you cannot map a multi-axis boxplot onto existing axes, but we could allow `ax` to be an array of axes. Would require:
- Checking that the number of axes passed is equal to the number of axes the boxplot
  wants to produce.
- Some documentation to make clear whether the axes get used in row or column order
  (or maybe it could support two-dimensional arrays of axes?)
