[mini-PR] Add particle tiling documentation (#1665)

* tiling wip

* do_tiling documentation

* Update Docs/source/running_cpp/parameters.rst

Co-authored-by: Luca Fedeli <luca.fedeli@for.unipi.it>

Co-authored-by: Luca Fedeli <luca.fedeli@for.unipi.it>

Docs/source/running_cpp/parameters.rst

@@ -355,6 +355,10 @@ Particle initialization
     particles are pushed in a standard way, using the specified pusher.
     (see the parameter ``<species_name>.zinject_plane`` below)
 
+* ``particles.do_tiling`` (`bool`) optional (default `true` if WarpX is compiled for GPUs, `false` otherwise)
+    Controls whether tiling ('cache blocking') transformation is used for particles.
+    Tiling should be on when using OpenMP and off when using GPUs.
+
 * ``<species_name>.species_type`` (`string`) optional (default `unspecified`)
     Type of physical species, ``"electron"``, ``"positron"``, ``"photon"``, ``"hydrogen"``.
     Either this or both ``mass`` and ``charge`` have to be specified.