mm: fix get_mctgt_type() kernel-doc

Convert the return values to an ReST list and tidy up the wording while
I'm touching it.

[akpm@linux-foundation.org: changes suggested by Randy]
[willy@infradead.org: another change suggested by Randy]
  Link: https://lkml.kernel.org/r/ZOUZtZizeQG7PcsM@casper.infradead.org
Link: https://lkml.kernel.org/r/20230818200630.2719595-3-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>