convert remaining style doc sections to be a logical header

doc/src/compute_sph_rho_atom.rst

@@ -35,7 +35,8 @@ LAMMPS.
 The value of the SPH density will be 0.0 for atoms not in the
 specified compute group.
 
-**Output info:**
+Output info
+"""""""""""
 
 This compute calculates a per-atom vector, which can be accessed by
 any command that uses per-atom values from a compute as input.  See
@@ -55,4 +56,7 @@ Related commands
 
 :doc:`dump custom <dump>`
 
-**Default:** none
+Default
+"""""""
+
+none