docs: refcount_t documentation

Some functions from refcount_t API provide different
memory ordering guarantees that their atomic counterparts.
This adds a document outlining these differences (
Documentation/core-api/refcount-vs-atomic.rst) as well as
some other minor improvements.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/include/linux/refcount.h b/include/linux/refcount.h
index e828658..4193c41 100644
--- a/include/linux/refcount.h
+++ b/include/linux/refcount.h
@@ -8,7 +8,7 @@
 #include <linux/kernel.h>
 
 /**
- * refcount_t - variant of atomic_t specialized for reference counts
+ * struct refcount_t - variant of atomic_t specialized for reference counts
  * @refs: atomic_t counter field
  *
  * The counter saturates at UINT_MAX and will not move once