Document that grid files override any grid variables set in input file

[ZedThree]

In this section it also states you can set ShiftAngle in the [mesh] section of the input file. This doesn't work for me -- is it still possible?

I'm not sure this is documented at all, but I think the grid variables are loaded either from a grid file (if that exsists) or from the BOUT.inp, but not both. If the grid file is used, any 'grid' variables in the [mesh] section of BOUT.inp are ignored (probably, based on a quick glance at src/mesh/data/ showing separate classes for loading grid data from grid file or options).

Originally posted by @johnomotani in https://github.com/boutproject/BOUT-dev/issues/2702#issuecomment-1525304493