Complete documentation of chardata handling

[pp-mo]

Additional to #6898

• review+tweak any public api + docstrings
• write a detailed subsection within the netcdf fileformat description
  • explain choices + controls, for both load+save
  • explain "choice" to have either char-arrays or string-arrays in cubes/coords
  • simple examples of loading
  • attachment of _Encoding attributes to control saving
  • explain/show how to correct "overlength when encoding" error
    • (i.e. use numpy.astype to lengthen the string dim created)