Documentation: Coccinelle: Improve command example for debugging patches

Modify Coccinelle documentation to clarify usage of make command to
run coccicheck on a folder.

Changes in v2:
- Give example of folder instead of file
- Add note
Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
Link: https://lore.kernel.org/r/20201126075730.w6brpeuviefmsxhl@adolinSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/dev-tools/coccinelle.rst

@@ -224,14 +224,21 @@ you may want to use::
 
     rm -f err.log
     export COCCI=scripts/coccinelle/misc/irqf_oneshot.cocci
-    make coccicheck DEBUG_FILE="err.log" MODE=report SPFLAGS="--profile --show-trying" M=./drivers/mfd/arizona-irq.c
+    make coccicheck DEBUG_FILE="err.log" MODE=report SPFLAGS="--profile --show-trying" M=./drivers/mfd
 
 err.log will now have the profiling information, while stdout will
 provide some progress information as Coccinelle moves forward with
 work.
 
+NOTE:
+
 DEBUG_FILE support is only supported when using coccinelle >= 1.0.2.
 
+Currently, DEBUG_FILE support is only available to check folders, and
+not single files. This is because checking a single file requires spatch
+to be called twice leading to DEBUG_FILE being set both times to the same value,
+giving rise to an error.
+
 .cocciconfig support
 --------------------