Index expression syntax

I wonder why syntax error is produced in this case:

postgres=# create index metaindex on foo using
gin(to_tsvector('english', x)||to_tsvector('english',y));
ERROR: syntax error at or near "||"
LINE 1: ...taindex on foo using gin(to_tsvector('english', x)||to_tsvec...
^
The error can be eliminated if extra surrounding parentheses are added:

postgres=# create index metaindex on foo using
gin((to_tsvector('english', x)||to_tsvector('english',y)));
CREATE INDEX

Postgresql documentations says:

CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ]/|name|/ ] ON/|table_name|/ [ USING/|method|/ ]
( {/|column_name|/ | (/|expression|/ ) } [ COLLATE/|collation|/ ] [/|opclass|/ ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] )
[ WITH (/|storage_parameter|/ =/|value|/ [, ... ] ) ]
[ TABLESPACE/|tablespace_name|/ ]
[ WHERE/|predicate|/ ]

/|expression:|/
An expression based on one or more columns of the table. The
expression usually must be written with surrounding parentheses, as
shown in the syntax. However, the parentheses can be omitted if the
expression has the form of a function call.

----------

So documentations states that sometimes it is possible to avoid
parentheses, but it is unclear why I have to use double parentheses...
I think that either grammar should be fixed, either documentation should
be updated.
/||/

--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company