Implement the documented -N and update the docs

[PaulWessel]

We advertised -N but had not code it up; this PR does that. Also, we stated that -As had not been implemented but it has been for a long time. I have now improved the man page to mention all of this. Closes #5246 but will not be merged until after 6.2.0 has been released, hence WIP.

[maxrjones]

The docs state that -A is required but grdfill without -A passes without warning (see below). Is -A required? If not, what is the default?

gmt grdclip @earth_relief_05m -R199:30/206/18/23 -Sa0/NaN -Gislands.nc
gmt grdfill islands.nc -Gnew.nc

[PaulWessel]

I think -A should be required. It is not obvious that anyone would expect a default method in this module. So I think we should check for it and complain if not given. Again, the docs are clear while the grdfill synopsis prints it as optional. I will fix this herein.

[PaulWessel]

I will do it in a separate PR.

[PaulWessel]

Hm. So either -A or -L are required. If you use -L then -A is not required. I think when that happens in other modules we list both -A and -L as optional but state that one must be selected, right?

[maxrjones]

Hm. So either -A or -L are required. If you use -L then -A is not required. I think when that happens in other modules we list both -A and -L as optional but state that one must be selected, right?

Yes and usually break with error if neither are provided.

[PaulWessel]

Should be OK here now, @meghanrjones.