doc: ug_implementation.rst: mention histfreq_base and hist_avg are pe…

…r-stream

In 35ec167 (Add functionality to change hist_avg for each stream
(CICE-Consortium#827), 2023-05-17), hist_avg was made into an array, allowing each
stream to individually be set to instantaneous or averaged mode. The
first paragraph of the "History" section of the user guide was updated,
but another paragraph a little below was not.

In 933b148 (Extend restart output controls, provide multiple frequency
options (CICE-Consortium#850), 2023-08-24), histfreq_base was also made into an array,
but the "History" section of the user guide was not updated.

Adjust the wording of the doc to reflect the fact that both hist_avg and
histfreq_base are per-stream. Also adjust the namelist excerpt to make
histfreq_base an array, and align hist_avg with it.

doc/source/user_guide/ug_implementation.rst

@@ -1197,9 +1197,9 @@ is a character string corresponding to ``histfreq`` or ‘x’ for none.
 files, no matter what the frequency is.) If there are no namelist flags
 with a given ``histfreq`` value, or if an element of ``histfreq_n`` is 0, then
 no file will be written at that frequency. The output period can be
-discerned from the filenames.  All history streams will be either instantaneous
-or averaged as specified by the ``hist_avg`` namelist setting and the frequency
-will be relative to a reference date specified by ``histfreq_base``.  Also, some
+discerned from the filenames.  Each history stream will be either instantaneous
+or averaged as specified by the corresponding entry in the ``hist_avg`` namelist array, and the frequency
+will be relative to a reference date specified by the corresponding entry in ``histfreq_base``.  Also, some
 Earth Sytem Models require the history file time axis to be centered in the averaging
 interval. The flag ``hist_time_axis`` will allow the user to chose ``begin``, ``middle``,
 or ``end`` for the time stamp. More information about how the frequency is 
@@ -1211,8 +1211,8 @@ For example, in the namelist:
 
   histfreq   = '1', 'h', 'd', 'm', 'y'
   histfreq_n =  1 ,  6 ,  0 ,  1 ,  1
-  histfreq_base = 'zero'
-  hist_avg = .true.,.true.,.true.,.true.,.true.
+  histfreq_base = 'zero','zero','zero','zero','zero'
+  hist_avg      = .true.,.true.,.true.,.true.,.true.
   f_hi = '1'
   f_hs = 'h'
   f_Tsfc = 'd'