Document open_dataset and open_mfdataset

Add the new functions to the I/O autosummary in api.rst and describe them in
io.rst, including why specifying an engine via mpas_tools.io.default_engine
avoids the CDF5 backend-sniffing crash and an example of setting the default
engine.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: xylar

conda_package/docs/api.rst

@@ -151,6 +151,8 @@ I/O
    :toctree: generated/
 
    write_netcdf
+   open_dataset
+   open_mfdataset
 
 Parallelism
 -----------

conda_package/docs/io.rst

@@ -26,3 +26,31 @@ Example usage:
     # Create a simple dataset
     ds = xr.Dataset({'foo': (('x',), [1, 2, 3])})
     write_netcdf(ds, 'output.nc')
+
+open_dataset and open_mfdataset
+===============================
+
+The :py:func:`mpas_tools.io.open_dataset()` and
+:py:func:`mpas_tools.io.open_mfdataset()` functions are thin wrappers around
+:py:func:`xarray.open_dataset()` and :py:func:`xarray.open_mfdataset()`.  They
+select the NetCDF ``engine`` from the module-level
+:py:data:`mpas_tools.io.default_engine` variable when an ``engine`` is not
+passed explicitly.
+
+This is useful because :py:func:`xarray.open_dataset()` otherwise sniffs the
+file for "magic bits" to auto-select a backend, and that probe can crash on
+``NETCDF3_64BIT_DATA`` (CDF5) files.  xarray provides no global way to set a
+default engine, so ``mpas_tools.io.default_engine`` offers a single,
+process-wide knob that applies to both reading and writing.
+
+Example usage:
+
+.. code-block:: python
+
+    import mpas_tools.io
+    from mpas_tools.io import open_dataset
+
+    # use the netcdf4 engine everywhere to avoid the CDF5 sniffing crash
+    mpas_tools.io.default_engine = 'netcdf4'
+
+    ds = open_dataset('mesh.nc')