Better docstring for Bokeh `marker` options

I think that, when the bokeh plotting backend is active, we support the Bokeh markers AND a subset of the Maplotlib markers thanks to this mapping in holoviews https://github.com/holoviz/holoviews/blob/67201d13e226f04b614ede4131c321fed5353edb/holoviews/plotting/bokeh/styles.py#L88-L100. I don't think that mapping is documented anywhere in HoloViews, but I found out about it looking at the code, and so I documented it here but not very clearly I guess.

_Originally posted by @maximlt in https://github.com/holoviz/hvplot/pull/1658#discussion_r3434306725_

Review the linked HoloViews styles.py marker mapping and locate the corresponding hvPlot marker docstring. Clarify which Bokeh markers and mapped Matplotlib markers are supported, then verify that the resulting docstring accurately reflects the mapping and renders clearly in the documentation.

Written by the indexing model from the issue text.