docs: add memory API reference

Add kernel-doc directive to parse and include doc comments from
include/exec/memory.h.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/devel/memory.rst

@@ -361,3 +361,8 @@ callbacks are called:
 - .impl.unaligned specifies that the *implementation* supports unaligned
   accesses; if false, unaligned accesses will be emulated by two aligned
   accesses.
+
+API Reference
+-------------
+
+.. kernel-doc:: include/exec/memory.h