Documentation: Coccinelle: document debug log handling

The current debug documentation does not mention that logs are printed
to stdout unless DEBUG_FILE is set. It also doesn't mention that
Coccinelle cannot overwrite debug files.

Document this behaviour in the examples and reference it in the
debugging section.

Signed-off-by: Benjamin Philip <benjamin.philip495@gmail.com>
Signed-off-by: Julia Lawall <julia.lawall@inria.fr>

Author: Benjamin-Philip

Documentation/dev-tools/coccinelle.rst

@@ -127,6 +127,18 @@ To enable verbose messages set the V= variable, for example::
 
    make coccicheck MODE=report V=1
 
+By default, coccicheck will print debug logs to stdout and redirect stderr to
+/dev/null. This can make coccicheck output difficult to read and understand.
+Debug and error messages can instead be written to a debug file instead by
+setting the ``DEBUG_FILE`` variable::
+
+    make coccicheck MODE=report DEBUG_FILE="cocci.log"
+
+Coccinelle cannot overwrite a debug file. Instead of repeatedly deleting a log
+file, you could include the datetime in the debug file name::
+
+    make coccicheck MODE=report DEBUG_FILE="cocci-$(date -Iseconds).log"
+
 Coccinelle parallelization
 --------------------------
 
@@ -208,11 +220,10 @@ include options matching the options used when we compile the kernel.
 You can learn what these options are by using V=1; you could then
 manually run Coccinelle with debug options added.
 
-Alternatively you can debug running Coccinelle against SmPL patches
-by asking for stderr to be redirected to stderr. By default stderr
-is redirected to /dev/null; if you'd like to capture stderr you
-can specify the ``DEBUG_FILE="file.txt"`` option to coccicheck. For
-instance::
+An easier approach to debug running Coccinelle against SmPL patches is to ask
+coccicheck to redirect stderr to a debug file. As mentioned in the examples, by
+default stderr is redirected to /dev/null; if you'd like to capture stderr you
+can specify the ``DEBUG_FILE="file.txt"`` option to coccicheck. For instance::
 
     rm -f cocci.err
     make coccicheck COCCI=scripts/coccinelle/free/kfree.cocci MODE=report DEBUG_FILE=cocci.err