mm/doc: editorial pass on page migration

Add Sphinx reference links to HMM and CPUSETS, and numerous small
editorial changes to make the page_migration.rst document more readable.
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20200902225247.15213-1-rcampbell@nvidia.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

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

+.. _cpusets:
+
 =======
 CPUSETS
 =======

Documentation/vm/hmm.rst

-.. hmm:
+.. _hmm:
 
 =====================================
 Heterogeneous Memory Management (HMM)