clarification for thresh arg of dataset.dropna() (#7481)

* clarification for thresh arg of dataset.dropna()

* Update xarray/core/dataset.py

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Co-authored-by: Mathias Hauser <mathause@users.noreply.github.com>

Author: pierreloicq

xarray/core/dataset.py

@@ -5474,7 +5474,7 @@ def dropna(
             - all : if all values are NA, drop that label
 
         thresh : int or None, optional
-            If supplied, require this many non-NA values.
+            If supplied, require this many non-NA values (summed over all the subset variables).
         subset : iterable of hashable or None, optional
             Which variables to check for missing values. By default, all
             variables in the dataset are checked.