Update docstring in api.py for open_mfdataset(), clarifying "chunks" …

…argument (#9121)

Per this discussion: #9119

xarray/backends/api.py

@@ -863,7 +863,8 @@ def open_mfdataset(
         In general, these should divide the dimensions of each dataset. If int, chunk
         each dimension by ``chunks``. By default, chunks will be chosen to load entire
         input files into memory at once. This has a major impact on performance: please
-        see the full documentation for more details [2]_.
+        see the full documentation for more details [2]_. This argument is evaluated
+        on a per-file basis, so chunk sizes that span multiple files will be ignored.
     concat_dim : str, DataArray, Index or a Sequence of these or None, optional
         Dimensions to concatenate files along.  You only need to provide this argument
         if ``combine='nested'``, and if any of the dimensions along which you want to