doc: ug_troubleshooting: clarify 'debug_model' description

In d6eb125 (Add new unit tests sumchk and bcstchk and update tests
(CICE-Consortium#606), 2021-06-09), the namelist variables
'debug_model_{i,j,iblk,task}' were added and 'debug_model' was changed
to print diagnostics for the grid point specified by those variables, if
they are defined, and fall back to (lonpnt(1), latpnt(1)) instead. The
"Diagnostic files" subsection of the  "Implementation" section of the
User guide was updated, but not the "Debugging hints" subsection of the
"Troubleshooting" section, where the various namelist flags and
subroutines useful for debugging are mentioned.

Adjust the description of 'debug_model' there, and also mention that in
contrast to 'print_points', this option outputs diagnostics several
times per time step. While at it, mention that 'print_points' output is
every 'diagfreq' time steps.

doc/source/user_guide/ug_troubleshooting.rst

@@ -138,7 +138,10 @@ Namelist flags
     Print diagnostics during block decomposition and distribution.
 
 `debug\_model` = true (**ice\_in**)
-    Print extended diagnostics for the first point associated with `print\_points`.
+    Print extended diagnostics for the grid point defined by ``debug_model_i``,
+    ``debug_model_j``, ``debug_model_iblk`` and ``debug_model_task``. If those are unset,
+    defaults to printing diagnostics for the point corresponding to coordinates ``lonpnt(1)``, ``latpnt(1)``.
+    These diagnostics are printed several times per time step.
 
 `debug\_model\_i` = integer (**ice\_in**)
     Defines the local i index for the point to be diagnosed with `debug\_model`.
@@ -167,7 +170,8 @@ Namelist flags
     also provides the local grid indices and block and processor numbers
     (`ip`, `jp`, `iblkp`, `mtask`) for these points, which can be used in
     to call *print\_state*. This option can be fairly slow,
-    due to gathering data from processors.
+    due to gathering data from processors. These diagnostics are written
+    once every ``diagfreq`` time steps.
 
 `conserv\_check` = true (**ice\_in**)
     Diagnoses conservation in various algorithms.