coccinelle: document why src/libc/ and src/test/ are excluded

For some of the directories it makes more sense to keep them
excluded from the coccinelle check. Specifically:
- libc: compatibility, no asserts or systemd headers yet
- test: uses NUL internally to test crashes etc

meson.build

@@ -2981,7 +2981,6 @@ if spatch.found()
                 'src/core/',
                 'src/import/',
                 'src/journal/',
-                'src/libc/',
                 'src/libsystemd/',
                 'src/libsystemd-network/',
                 'src/network/',
@@ -2989,6 +2988,9 @@ if spatch.found()
                 'src/nss-systemd/',
                 'src/resolve/',
                 'src/shared/',
+                # libc/ has no assert() or systemd-headers so leave it
+                'src/libc/',
+                # test/ has some deliberate wonky pointers, just leave excluded
                 'src/test/',
         ]