BUG #8335: trim() un-document behaviour

The following bug has been logged on the website:

Bug reference: 8335
Logged by: Jov
Email address: amutu(at)amutu(dot)com
PostgreSQL version: 9.2.4
Operating system: suse 10 linux 64
Description:

in the postgresql doc 9.4,I find the trim() function like this:

trim([leading | trailing | both] [characters] from string)

so the trim should be pass only one argument with some optional prefix。but I
find the following calls with two argument is successfull but the results is
unexpected and wired：

##first call
postgres=# select trim(trailing ‘/’, ‘fasd/’);
rtrim
——-

(1 row)
-----!!!note: it return titile is rtrim----

## second call
postgres=# select trim(‘/’, ‘fasd/’)
;
btrim
——-

(1 row)
-----!!!note: it return titile is btrim----

it seems trim is transform to r、b、l trim internal，but the above call should
return error or it may produce un-expect results