Mention sig_figs in Stan CSV file format doc

Author: avehtari

src/cmdstan-guide/command_line_options.qmd

@@ -57,7 +57,7 @@ the executable exits with the following message:
 Exception: variable does not exist; processing stage=data initialization; variable name=y; base type=int (in 'examples/bernoulli/bernoulli.stan', line 3, column 2 to column 28)
 ```
 
-## Output control arguments
+## Output control arguments {#output-control.section}
 
 The `output` keyword is used to specify non-default options for
 output files and messages written to the terminal window.

src/cmdstan-guide/stan_csv_apdx.qmd

@@ -13,8 +13,10 @@ For the Stan CSV files, data is strictly numerical,
 however, possible values include both positive and negative
 infinity and "Not-a-Number" which are represented as
 the strings `NaN`, `inf`, `+inf`, `-inf`.
-All other values are written in decimal notation with
-at most 6 digits of precision.
+All other values are written in decimal notation by default with
+at most 6 digits of precision. The number of significant digits 
+written can be controlled with argument `sig_figs=<int>`. See more 
+in [Output control arguments section](command_line_options.qmd).
 
 Stan CSV files have a header row containing the column names.
 They also make extensive use of CSV comments,