DOCS: clarify meaning of merge_streams option (#664)

docs/render/format_code_cells.md

@@ -84,17 +84,18 @@ This can be set to:
 (render/output/group-stderr-stdout)=
 ### Group into single streams
 
-Cells may print multiple things to `stdout` and `stderr`.
+Cells may print multiple things to `stdout` (and/or to `stderr`).
 For example, if a cell prints status updates throughout its execution, each of these is often printed to `stdout`.
 By default, these outputs may be split across multiple items, and will be rendered as separate "chunks" in your built documentation.
 
-If you'd like each of the outputs in `stderr` and `stdout` to be merged into a single stream for each, use the following configuration:
+If you'd like each of the outputs in `stdout` to be merged into a single stream for each, use the following configuration (see {ref}`config/intro`):
 
 ```python
 nb_merge_streams = True
 ```
 
-This will ensure that all `stderr` and `stdout` outputs are merged into a single group.
+This will ensure that all `stdout` outputs are merged into a single group; likewise, all
+`stderr` outputs will be merged into single (separate) group.
 This also makes cell outputs more deterministic.
 Normally, slight differences in timing may result in different orders of `stderr` and `stdout` in the cell output, while this setting will sort them properly.
 

myst_nb/core/config.py

@@ -285,7 +285,7 @@ def __post_init__(self):
         default=False,
         metadata={
             "validator": instance_of(bool),
-            "help": "Merge stdout/stderr execution output streams",
+            "help": "Merge all stdout execution output streams; same with stderr",
             "sections": (
                 Section.global_lvl,
                 Section.file_lvl,