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)?
@agijsberts The DatetimeConverter in pandas was speed up by you in that PR, but is this also faster as matplotlib's built in converter? (what you would use when not importing pandas?)