Fixed sphinx error when building netcdf save docs

Building the docs currently produces a non-fatal error:
```
iris/lib/iris/fileformats/netcdf.py:docstring of iris.fileformats.netcdf.save:7: ERROR: Unexpected indentation.
```
This PR fixes it and also adds a docstring to ```iris.fileformats.netcdf.Saver``` to replace the odd looking 'None' in the docs.