remove some outdated occurrences of passing indexes={}

docs/usage.md

@@ -205,7 +205,7 @@ But before we combine our data, we might want to consider loading some variables
 Whilst the values of virtual variables (i.e. those backed by `ManifestArray` objects) cannot be loaded into memory, you do have the option of opening specific variables from the file as loadable lazy numpy/dask arrays, just like `xr.open_dataset` normally returns. These variables are specified using the `loadable_variables` argument:
 
 ```python
-vds = open_virtual_dataset('air.nc', loadable_variables=['air', 'time'], indexes={})
+vds = open_virtual_dataset('air.nc', loadable_variables=['air', 'time'])
 ```
 ```python
 <xarray.Dataset> Size: 31MB
@@ -228,8 +228,8 @@ You can see that the dataset contains a mixture of virtual variables backed by `
 Loading variables can be useful in a few scenarios:
 1. You need to look at the actual values of a multi-dimensional variable in order to decide what to do next,
 2. You want in-memory indexes to use with ``xr.combine_by_coords``,
-3. Storing a variable on-disk as a set of references would be inefficient, e.g. because it's a very small array (saving the values like this is similar to kerchunk's concept of "inlining" data),
-4. The variable has encoding, and the simplest way to decode it correctly is to let xarray's standard decoding machinery load it into memory and apply the decoding.
+3. Storing a variable on-disk as a set of references would be inefficient, e.g. because each chunk is very small (saving the values like this is similar to kerchunk's concept of "inlining" data),
+4. The variable has complicated encoding, and the simplest way to decode it correctly is to let xarray's standard decoding machinery load it into memory and apply the decoding.
 
 ### CF-encoded time variables
 
@@ -240,7 +240,6 @@ vds = open_virtual_dataset(
     'air.nc',
     loadable_variables=['air', 'time'],
     decode_times=True,
-    indexes={},
 )
 ```
 ```python
@@ -341,7 +340,6 @@ In future we would like for it to be possible to just use `xr.open_mfdataset` to
         concat_dim=['time'],
         coords='minimal',
         compat='override',
-        indexes={},
     )
 
 but this requires some [upstream changes](https://github.com/TomNicholas/VirtualiZarr/issues/35) in xarray.