Undocumented functions

Some days ago Tom Lane said ...

SELECT events & 4 != 0 AS can_upd, events & 8 != 0 AS can_ins, events & 16
!= 0 AS can_del FROM
pg_catalog.pg_relation_is_updatable('_pessoa'::regclass, false) t(events);

Well, I didn't find that function on DOCs and then I thought, are there
other functions which are not documented ? Why ?
Then if I get all functions from pg_catalog
select string_agg(distinct format('"%s"',proname),',') from pg_proc where
pronamespace::regnamespace::text = 'pg_catalog'

using PowerShell declare a variable, on the SGML folder I use ...

$vv = (result of that select)
foreach ($v in $vv) {if (!((Get-Content . | %{$_ -match $v}) -contains
$true)) {
Write-Host $v}}

I'll get all functions which are on pg_catalog but not on SGML files, so
are not documented.

Example, elem_contained_by_range is not documented. I know I can use
select 2 <@ '[1,3]'::int4range
But why is that function not documented ?
select elem_contained_by_range(2,'[1,3]'::int4range);

And what other functions are cool to use but are not documented ?

regards
Marcos