Minor tweaks to the wording of the theory section (BLAST-WarpX#6363)

This incorporates feedback from the rest of the WarpX team

Author: RemiLehe

Docs/source/theory/cold_fluid_model.rst

@@ -1,7 +1,7 @@
 .. _theory-cold-fluid-model:
 
-Cold Relativistic Fluid Model
-=============================
+Fluid Representation
+====================
 
 An alternate to the representation of the plasma as macroparticles, is the cold relativistic fluid model.
 The cold relativistic fluid model is typically faster to compute than

Docs/source/theory/intro.rst

@@ -6,7 +6,7 @@ Overview
 .. _theory-pic:
 
 WarpX simulates the **self-consistent** evolution of **particle species** (e.g., electrons, ions, etc.) in the presence of **electric and magnetic fields**.
-In this context, *self-consistent* indicates that the particle dynamics are influenced by the fields, while the fields themselves evolve in response to the particles' changing charge and current density.
+In this context, *self-consistent* indicates that the particle dynamics are influenced by the fields, while the fields themselves evolve in response to the particles' changing charge and current densities.
 
 The fields are represented on a **discrete spatial grid** (see :ref:`theory-grid`).
 The species are most commonly represented by **discrete macroparticles** moving continuously through the grid, but can also be represented as **fluids** discretized on a grid (see :ref:`theory-species_representations`).

Docs/source/theory/models_algorithms/electromagnetic_pic.rst

@@ -1,9 +1,9 @@
 .. _theory-em-pic:
 
-Fully electromagnetic PIC
-=========================
+Electromagnetic PIC
+===================
 
-In the *fully electromagnetic Particle-In-Cell method* :cite:p:`pt-Birdsalllangdon,pt-HockneyEastwoodBook`,
+In the *electromagnetic Particle-In-Cell method* :cite:p:`pt-Birdsalllangdon,pt-HockneyEastwoodBook`,
 the fields are updated using Maxwell's equations:
 
 .. math::