particles: fix bugs in cl_particles_quake

particles: fix bugs in cl_particles_quake

There was a segfault when CL_NewQuakeParticle->CL_NewParticle returned
NULL, and a missing break; in case pt_snow: in R_RenderParticles causing
fall-through to pt_explode.
Introduced in 58dc7879146968c2c07e1ffd2c521487a5e768d9, I missed them in
code review, oops.

Could have just added checks for NULL in CL_ParticleExplosion but these
kinds of exceptions can get messy if several accumulate so it seemed
better to merge them into the API which also made NULL checks redundant.

Changes the particle_t comments to doxygen comments.
Fixes indenting.

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>