fix doc for coarsening (ECP-WarpX#4588)

Docs/source/usage/parameters.rst

@@ -2602,8 +2602,8 @@ In-situ capabilities can be used by turning on Sensei or Ascent (provided they a
     Only works with ``<diag_name>.format = plotfile``.
 
 * ``<diag_name>.coarsening_ratio`` (list of `int`) optional (default `1 1 1`)
-    Reduce size of the field output by this ratio in each dimension.
-    (This is done by averaging the field over 1 or 2 points along each direction, depending on the staggering).
+    Reduce size of the selected diagnostic fields output by this ratio in each dimension.
+    (For a ratio of N, this is done by averaging the fields over N or (N+1) points depending on the staggering).
     If ``blocking_factor`` and ``max_grid_size`` are used for the domain decomposition, as detailed in
     the :ref:`domain decomposition <usage_domain_decomposition>` section, ``coarsening_ratio`` should be an integer
     divisor of ``blocking_factor``. If ``warpx.numprocs`` is used instead, the total number of cells in a given