Extend marker drawing functionality & docstr

[snototter]

• Draw single characters, similar to draw_marker with a valid bg_color and border.
  • The goal would be to have a single letter/short text over a white circle.
  • This would allow more flexible position labeling for the user, e.g. draw_char('A', position=(x,y), text_style=..., bg_color=..., bg_border=...)
  • Examples: 'A', '23', 'LE' (for "left eye")
  • Idea: Compute circle diameter from font metrics
• Additionally, fix code example for MarkerStyle: it is missing the bg_... keywords