DOC: update docs on direct plotting with matplotlib (GH8614)

Closes #8614 

- this adds a warning that you now have to use `to_pydatetime` as direct plotting with a DatetimeIndex does not work anymore (see #8614)
- I also removed for now the note on speed and explanation of the registered formatters. @agijsberts could you shed some light on this?
     - "*The speed up for large data sets only applies to pandas 0.14.0 and later.*" Why only for pandas 0.14 or later? And from where does this speed-up come from?
     - "*thereby extending date and time support to practically all plot types available in matplotlib*" -> but if you plot directly with matplotlib, I think you don't use the pandas registered formatters? So that sentence seems not fully correct, is that possible? And isn't that the reason for the possible speed-up (matplotlib defaults formatter being faster as pandas' formatter)?
"*extending date and time support to practically all plot types available in matplotlib*" -> @agijsberts but matplotlib by default has also a datetime converter?