docs: cgroup-v1: make swap extension subsections subsections

Subsections text of swap extension section is marked up as bold text,
whereas making them proper subsection is more appropriate.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/admin-guide/cgroup-v1/memory.rst

@@ -244,15 +244,17 @@ In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap.
 By using the memsw limit, you can avoid system OOM which can be caused by swap
 shortage.
 
-**why 'memory+swap' rather than swap**
+2.4.1 why 'memory+swap' rather than swap
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
 to move account from memory to swap...there is no change in usage of
 memory+swap. In other words, when we want to limit the usage of swap without
 affecting global LRU, memory+swap limit is better than just limiting swap from
 an OS point of view.
 
-**What happens when a cgroup hits memory.memsw.limit_in_bytes**
+2.4.2. What happens when a cgroup hits memory.memsw.limit_in_bytes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 When a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-out
 in this cgroup. Then, swap-out will not be done by cgroup routine and file