add cross reference to directive `default_border` from command `border` (#4273)
add cross reference to directive `default_border` from command `border` (#4273)

Added a couple of examples to make usage clearer with a cursory glance.
Airblader/i3-original,Airblader/i3-original,stapelberg/i3,Airblader/i3,i3/i3,Airblader/i3-original,Airblader/i3-original,Airblader/i3,stapelberg/i3,Airblader/i3,i3/i3,Airblader/i3,stapelberg/i3,i3/i3,Airblader/i3-original,stapelberg/i3,i3/i3