DOC: fixes multiple errors in alter table doc

Hi Hacker,

While working on a patch these days, my eyes are on the “alter table” doc,
and found multiple errors:

1. Several sub-commands are missed in the top “action” list:

* ALTER COLUMN SET <sequence-option>
* ALTER COLUMN RESTART
* RENAME
* SET SCHEMA
* ATTACH PARTITION
* DETACH PARTITION
* MERGE PARTITION
* SPLIT PARTITION

2. In sub-command details section, "ADD COLUMN [ IF NOT EXISTS ]” missed
“[]" with “COLUMN”, which is misleading, because “COLUMN” is actually
optional.

3. For all “alter column” sub-commands, "ALTER [ COLUMN ]” are omitted,
which is also confusing, because none of other sub-commands omit their
prefix part.

This patch fixed all these issue.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/