Undocumented array_val[generate_series(...)] functionality?

Hey,

A post over in Reddit had an expression form I've never seen before:

select (array[1,2,3,4]::integer[])[generate_series(1, 3)];
===
1
2
3

Looking at subscripting in the SQL syntax this example doesn't seem to be
documented.

https://www.postgresql.org/docs/current/sql-expressions.html#SQL-EXPRESSIONS-SUBSCRIPTS

Is it documented somewhere else, and, regardless, shouldn't it be
documented in the linked location?

I get this is basically a variant of the slice syntax [n:m], and it is
working as expected, but nothing indicates I can put an SRF inside the
brackets.

David J.