Additional doc fixes for configurable TOAST compression.

Additional doc fixes for configurable TOAST compression.

The grammar changes in commit bbe0a81db69bd10bd166907c3701492a29aca294
allow SET COMPRESSION to be used with ALTER MATERIALIZED VIEW as
well as with ALTER TABLE, so update those docs to say that it works.

Also, update the documentation for the pg_column_compression()
to explain that it will return NULL when there's no relevant value.

Patch by me, per concerns from Michael Paquier.

Discussion: http://postgr.es/m/CA+Tgmob9h5u4iNL9KM0drZgkY-JL4oCVW0dWrMqtLPQ1zHkquA@mail.gmail.com