Add information in thread to documentation

[eugeneburger]

Add information on the restriction described here to the documentation

https://www.pmel.noaa.gov/maillists/tmap/ferret_users/fu_2019/msg01105.html

[ACManke]

See also issue #51 where comments include example code

[seasage]

I've updated the documentation with the work-around as Karl wrote it in #51 as an example here under /PLOT/GRATICULE: https://ferret.lark.pmel.noaa.gov/documentation/users-guide/commands-reference/PLOT