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@adolin


Signed-off-by: Jonathan Corbet <corbet@lwn.net>