Improve docs for atomic and load/store operations. (#2437)

- Move atomic_cas and atomic_xchg to "atomic ops" section of
   documentation.
 - Don't talk about the `cmp` operand for operations which don't have
   it.
 - Document the `sem` operand.
 - :code:`foo` and ``foo`` don't work inside a :type: annotation,
   apparently.  (They are rendered literally, instead of being treated
   as a formatting command.)  Get rid of them.
- Format the bulleted lists in the load/store operations as intended.

docs/python-api/triton.language.rst

@@ -63,8 +63,6 @@ Memory Ops
 
     load
     store
-    atomic_cas
-    atomic_xchg
 
 
 Indexing Ops
@@ -129,10 +127,11 @@ Atomic Ops
     :toctree: generated
     :nosignatures:
 
-    atomic_cas
     atomic_add
+    atomic_cas
     atomic_max
     atomic_min
+    atomic_xchg
 
 
 Comparison ops