Clarify reduced diagnostics

This commit aims to fix a potential misunderstanding that reduced diagnostics
are of lower quality and not for production purposes by emphasizing that
"reduced" indicates in-situ reduction operations.

Docs/source/usage/parameters.rst

@@ -2680,7 +2680,7 @@ WarpX has five types of diagnostics:
 ``TimeAveraged`` diagnostics only allow field data, which they output after averaging over a period of time,
 ``BackTransformed`` diagnostics are used when running a simulation in a boosted frame, to reconstruct output data to the lab frame,
 ``BoundaryScraping`` diagnostics are used to collect the particles that are absorbed at the boundary, throughout the simulation, and
-``ReducedDiags`` allow the user to compute some reduced quantity (particle temperature, max of a field) and write a small amount of data to text files.
+``ReducedDiags`` enable users to compute specific reduced quantities, such as particle temperature, energy histograms, or maximum field values, and efficiently save this in-situ analyzed data to files.
 Similar to what is done for physical species, WarpX has a class Diagnostics that allows users to initialize different diagnostics, each of them with different fields, resolution and period.
 This currently applies to standard diagnostics, but should be extended to back-transformed diagnostics and reduced diagnostics (and others) in a near future.
 
@@ -3064,23 +3064,22 @@ In addition to their usual attributes, the saved particles have
 Reduced Diagnostics
 ^^^^^^^^^^^^^^^^^^^
 
-``ReducedDiags`` allow the user to compute some reduced quantity (particle temperature, max of a field) and write a small amount of data to text files.
+``ReducedDiags`` enable users to compute specific reduced quantities, such as particle temperature, energy histograms, or maximum field values, and efficiently save this in-situ analyzed data to files.
+This shifts analysis from post-processing to runtime calculation of reduction operations (average, maximum, ...) and can greatly save disk space when "raw" particle and field outputs from `FullDiagnostics` can be avoided in favor of single values, 1D or 2D data at possibly even higher time resolution.
 
 * ``warpx.reduced_diags_names`` (`strings`, separated by spaces)
-    The names given by the user of simple reduced diagnostics.
-    Also the names of the output `.txt` files.
-    This reduced diagnostics aims to produce simple outputs
-    of the time history of some physical quantities.
+    A list of user-given names for reduced diagnostics.
+    By default, these names are also prefixing the names of output files.
     If ``warpx.reduced_diags_names`` is not provided in the input file,
-    no reduced diagnostics will be done.
+    no reduced diagnostics will be activated during the run.
     This is then used in the rest of the input deck;
     in this documentation we use ``<reduced_diags_name>`` as a placeholder.
 
 * ``<reduced_diags_name>.type`` (`string`)
     The type of reduced diagnostics associated with this ``<reduced_diags_name>``.
     For example, ``ParticleEnergy``, ``FieldEnergy``, etc.
     All available types are described below in detail.
-    For all reduced diagnostics,
+    For all reduced diagnostics that are writing tabular data into text files,
     the first and the second columns in the output file are
     the time step and the corresponding physical time in seconds, respectively.