Add information about generating particles for GPU codes to the docs (#4760)

The proposed changes:
- [ ] fix a bug or incorrect behavior in AMReX
- [ ] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [x] include documentation in the code and/or rst files, if appropriate

---------

Co-authored-by: Axel Huebl <[email protected]>

Author: atmyers

Docs/sphinx_documentation/source/Particle.rst

@@ -309,6 +309,18 @@ particles, if the processes generate particles they don't own (for example, if
 the particle positions are perturbed from the cell centers and thus end up
 outside their parent grid).
 
+.. note::
+
+   The above code snippet, which successively calls :cpp:`push_back` on the particle
+   vectors, assumes you have compiled AMReX for CPU execution.
+   For GPU codes, one can either generate particles on the host and copy them
+   to the device, or generate the particles directly on the GPU. For the first
+   approach, please see the sample code `here <https://github.com/AMReX-Codes/amrex/blob/development/Tests/Particles/Redistribute/main.cpp#L81>`__.
+   For an example of generating a variable number of particles in each cell
+   directly on the GPU, please see
+   `this <https://github.com/AMReX-Codes/amrex-tutorials/blob/main/ExampleCodes/Particles/ElectromagneticPIC/Source/EMParticleContainerInit.cpp#L48>`__
+   Electromagnetic Particle-in-Cell tutorial
+
 .. _sec:Particles:Runtime:
 
 Adding particle components at runtime