Add explanation of leveling grid to documentation

[kaysond]

I had a hard time finding the below. Can it be added to the documentation? Either G29 or M420?

@aplocher — 0,0 is the mesh point closest to the smallest XY position in the measured bounds. So, if the bounds specified for G29 were given as L20 F20 R180 B180 then...

X020 Y180     X100 Y180     X180 Y180
   0    2        1    2        2    2


X020 Y100     X100 Y100     X180 Y100
   0    1        1    1        2    1


X020 Y020     X100 Y020     X180 Y020
   0    0        1    0        2    0

Originally posted by @thinkyhead in https://github.com/MarlinFirmware/Marlin/issues/5936#issuecomment-395917430