pgindent: more doc updates for skipping __asm__ files

src/tools/pgindent/README

@@ -81,8 +81,8 @@ see:
 Notes about excluded files
 --------------------------
 
-src/include/storage/s_lock.h is excluded because it contains assembly code
-that pgindent tends to mess up.
+src/include/storage/s_lock.h and src/include/port/atomics/ are excluded
+because they contains assembly code that pgindent tends to mess up.
 
 src/include/snowball/libstemmer/ and src/backend/snowball/libstemmer/
 are excluded because those files are imported from an external project,