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.

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