Document the HoloViews plot `clim_percentile`

[maximlt]

xarray has a robust boolean option that when set to True use the 2nd and 98th percentiles of the data to compute the color limits. HoloViews has a similar option named clim_percentile, that is actually even more flexible as it behaves like robust if set to True but also accepts int/float numbers to specify another cut-off percentile (e.g. 10 instead of 2). This option should at least be documented in the gridded data guide.

Note that until HoloViews 1.15.1 this option was broken in HoloViews: https://github.com/holoviz/holoviews/pull/5495