Fix settings io_format documentation. (#3718)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

Author: GuySten

docs/source/io_formats/settings.rst

@@ -277,6 +277,15 @@ ignored for all run modes other than "eigenvalue".
 
   *Default*: 1
 
+------------------------------
+``<ifp_n_generation>`` Element
+------------------------------
+
+The ``<ifp_n_generation>`` element indicates the number of generations to
+consider for the Iterated Fission Probability method.
+
+  *Default*: 10
+
 ----------------------
 ``<inactive>`` Element
 ----------------------
@@ -402,7 +411,25 @@ then, OpenMC will only use up to the :math:`P_1` data.
 ``<max_history_splits>`` Element
 --------------------------------
 
-The ``<max_history_splits>`` element indicates the number of times a particle can split during a history.
+The ``<max_history_splits>`` element indicates the number of times a particle
+can split during a history.
+
+  *Default*: 1000
+
+-----------------------------
+``<max_secondaries>`` Element
+-----------------------------
+
+The ``<max_secondaries>`` element indicates the maximum secondary bank size.
+
+  *Default*: 10000
+
+------------------------
+``<max_tracks>`` Element
+------------------------
+
+The ``<max_tracks>`` element indicates the maximum number of tracks written to a
+track file (per MPI process).
 
   *Default*: 1000
 
@@ -1378,6 +1405,15 @@ has the following attributes/sub-elements:
               for fixed source and small criticality calculations, but is very
               optimistic for highly coupled full-core reactor problems.
 
+-------------------------------------
+``<uniform_source_sampling>`` Element
+-------------------------------------
+
+The ``<uniform_source_sampling>`` element indicates whether to sample among
+multiple sources uniformly, applying their strengths as weights to sampled
+particles.
+
+  *Default*: False
 
 ------------------------
 ``<ufs_mesh>`` Element
@@ -1390,6 +1426,16 @@ Agency Monte Carlo Performance Benchmark Problem," Proceedings of *Physor 2012*,
 Knoxville, TN (2012). The mesh should cover all possible fissionable materials
 in the problem and is specified using a :ref:`mesh_element`.
 
+-------------------------------
+``<use_decay_photons>`` Element
+-------------------------------
+
+The ``<use_decay_photons>`` element indicates whether to produce decay photons
+from neutron reactions instead of prompt photons. This is used in conjunction
+with the direct 1-step method for shutdown dose rate calculations.
+
+  *Default*: False
+
 .. _verbosity:
 
 -----------------------
@@ -1620,3 +1666,21 @@ following sub-elements/attributes:
 
   The ``weight_windows_file`` element has no attributes and contains the path to
   a weight windows HDF5 file to load during simulation initialization.
+
+-------------------------------
+``<weight_windows_on>`` Element
+-------------------------------
+
+  The ``weight_windows_on`` element indicates whether weight windows are
+  enabled.
+
+  *Default*: False
+
+----------------------------------
+``<write_initial_source>`` Element
+----------------------------------
+
+  The ``write_initial_source`` element indicates whether to write the initial
+  source distribution to file.
+
+  *Default*: False