add some documentation and look into ncdataset.delete vs. ncgeodataset.d...

This has the doc patches in the other pull request--I don't know git well enough to separate them.

While I was looking at the docs, I tried a geo.delete and had problems.  I doubt this patch is the best way to resolve it, since it doesn't completely fix it.  It looks like this sort of problem would extend to any void-return obj.netcdf.methods accessed through the ncgeodataset.subsref stuff.

I wonder if undeleted ncgeodatasets have caused my matlab to crash while I've been experimenting.

    url = 'http://dods.mbari.org/cgi-bin/nph-nc/data/ssdsdata/deployments/m1/200810/OS_M1_20081008_TS.nc'
    geo = ncgeodataset(url)
    nm = geo.standard_name('sea_water_temperature')
    geo.netcdf.close  % fails, but works post-patch
    geo.delete % fails, fails silently and returns 0 post-patch
    cf = cfdataset(url)
    cf.delete
    cf = cfdataset(url)
    cf.netcdf.close %works
    nc = ncdataset(url)
    nc.netcdf.close % works
    nc = ncdataset(url)
    nc.delete  % works