BUG #13038: Unobvious snapshotting behavior on altered tables

The following bug has been logged on the website:

Bug reference: 13038
Logged by: Alexey Bashtanov
Email address: bashtanov(at)imap(dot)cc
PostgreSQL version: 9.4.1
Operating system: Linux CentOS 6.5 x64
Description:

Hello!

On truncate statement docs page there is a nice pink warning
http://www.postgresql.org/docs/9.4/static/sql-truncate.html#AEN85039

This warning also affects some table-rewriting ALTER TABLE statement, see
http://johtopg.blogspot.com.br/2014/11/postgresql-gotcha-of-week-week-45_6.html

The problem is
1) This is undocumented. Please document which commands can lead to this
behavior.
2) Not all the table-rewriting statements can show an empty snapshot. ALTER
TABLE ADD COLUMN, ALTER TABLE SET WITH/WITHOUT OIDS and TRUNCATE surely can.
ALTER TABLE SET TABLESPACE, CLUSTER, VACUUM FULL probably cannot. What is
the difference between the commands in the first list and in the second one?
Maybe this annoying behavior can be fixed for all DDL statements?

Best Regards
Alexey Bashtanov