man d.mon: mention why not all monitors are actually listed sometimes

[jidanni]

On https://grass.osgeo.org/grass79/manuals/d.mon.html mention that even though https://grass.osgeo.org/grass79/manuals/wxGUI.html#map-display-window looks like https://grass.osgeo.org/grass79/manuals/d_mon_wx0.png this will not list it:

$ d.mon -l
No monitors running.

If one wants to list it, use

$ wmctrl -l
0x01c0002b  0 jidanni5 GRASS GIS Map Display: 1 - newLocation/PERMANENT <<< there it is!
0x01c00024  0 jidanni5 GRASS GIS 7.8.1 Layer Manager

[wenzeslaus]

In addition to that, perhaps a better discussion about how d.mon and Map Display are different is needed in the manual page.