documentation: coccinelle: Improve command example for make C={1,2}

Modify coccinelle documentation to further clarify
the usage of the makefile C variable by coccicheck.
Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
Acked-by: Julia Lawall <julia.lawall@inria.fr>
Link: https://lore.kernel.org/r/20200811002350.5553-1-sylphrenadin@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/dev-tools/coccinelle.rst

@@ -175,13 +175,20 @@ For example, to check drivers/net/wireless/ one may write::
     make coccicheck M=drivers/net/wireless/
 
 To apply Coccinelle on a file basis, instead of a directory basis, the
-following command may be used::
+C variable is used by the makefile to select which files to work with.
+This variable can be used to run scripts for the entire kernel, a
+specific directory, or for a single file.
 
-    make C=1 CHECK="scripts/coccicheck"
+For example, to check drivers/bluetooth/bfusb.c, the value 1 is
+passed to the C variable to check files that make considers
+need to be compiled.::
 
-To check only newly edited code, use the value 2 for the C flag, i.e.::
+    make C=1 CHECK=scripts/coccicheck drivers/bluetooth/bfusb.o
 
-    make C=2 CHECK="scripts/coccicheck"
+The value 2 is passed to the C variable to check files regardless of
+whether they need to be compiled or not.::
+
+    make C=2 CHECK=scripts/coccicheck drivers/bluetooth/bfusb.o
 
 In these modes, which work on a file basis, there is no information
 about semantic patches displayed, and no commit message proposed.