docs: RCU: Convert rcuref.txt to ReST

- Add a SPDX header;
- Adjust document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add it to RCU/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/RCU/index.rst

@@ -18,6 +18,7 @@ RCU concepts
    whatisRCU
    rcu
    rculist_nulls
+   rcuref
    torture
    listRCU
    NMI-RCU

Documentation/RCU/rcuref.rst

@@ -1,4 +1,8 @@
-Reference-count design for elements of lists/arrays protected by RCU.
+.. SPDX-License-Identifier: GPL-2.0
+
+====================================================================
+Reference-count design for elements of lists/arrays protected by RCU
+====================================================================
 
 
 Please note that the percpu-ref feature is likely your first
@@ -12,7 +16,8 @@ please read on.
 Reference counting on elements of lists which are protected by traditional
 reader/writer spinlocks or semaphores are straightforward:
 
-CODE LISTING A:
+CODE LISTING A::
+
     1.					    2.
     add()				    search_and_reference()
     {					    {
@@ -46,7 +51,8 @@ search_and_reference() could potentially hold reference to an element which
 has already been deleted from the list/array.  Use atomic_inc_not_zero()
 in this scenario as follows:
 
-CODE LISTING B:
+CODE LISTING B::
+
     1.					    2.
     add()				    search_and_reference()
     {					    {
@@ -82,7 +88,8 @@ search_and_reference() code path.  In such cases, the
 atomic_dec_and_test() may be moved from delete() to el_free()
 as follows:
 
-CODE LISTING C:
+CODE LISTING C::
+
     1.					    2.
     add()				    search_and_reference()
     {					    {
@@ -130,7 +137,7 @@ the eventual invocation of kfree(), which is usually not a problem on
 modern computer systems, even the small ones.
 
 In cases where delete() can sleep, synchronize_rcu() can be called from
-delete(), so that el_free() can be subsumed into delete as follows:
+delete(), so that el_free() can be subsumed into delete as follows::
 
     4.
     delete()