Minor fixes for ALTER TABLE documentation.

Etsuro Fujita
Fix two bugs in WAL-logging of GIN pending-list pages.

In writeListPage, never take a full-page image of the page, because we
have all the information required to re-initialize in the WAL record
anyway. Before this fix, a full-page image was always generated, unless
full_page_writes=off, because when the page is initialized its LSN is
always 0. In stable-branches, keep the code to restore the backup blocks
if they exist, in case that the WAL is generated with an older minor
version, but in master Assert that there are no full-page images.

In the redo routine, add missing "off++". Otherwise the tuples are added
to the page in reverse order. That happens to be harmless because we
always scan and remove all the tuples together, but it was clearly wrong.
Also, it was masked by the first bug unless full_page_writes=off, because
the page was always restored from a full-page image.

Backpatch to all supported versions.
Add support for wrapping to psql's "extended" mode. This makes it very
feasible to display tables that have both many columns and some large
data in some columns (such as pg_stats).

Emre Hasegeli with review and rewriting from Sergey Muraviov and
reviewed by Greg Stark
Remove unnecessary cast causing a warning

Incidentally, I reversed the two names in the earlier commit. The
original author was Sergey Muraviov and the reviewer was Emre
Hasegeli.