BUG #16946: It may be that the content of the document is missing

The following bug has been logged on the website:

Bug reference: 16946
Logged by: 强 魏
Email address: 1726002692@qq.com
PostgreSQL version: 13.2
Operating system: centos
Description:

No information about the function pg_get_function_arg_default was found in
the official manual.
user:postgres@db:postgres[[local]:5432]#select version();
+-----------------------------------------------------------------------------+
|                                   version                                 
 |
+-----------------------------------------------------------------------------+
| PostgreSQL 13.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 7.3.0,
64-bit |
+-----------------------------------------------------------------------------+
(1 row)

Time: 0.664 ms
user:postgres@db:postgres[[local]:5432]#\df pg_get_function_arg_default
                                     List of functions
+------------+-----------------------------+------------------+---------------------+------+
|   Schema   |            Name             | Result data type | Argument
data types | Type |
+------------+-----------------------------+------------------+---------------------+------+
| pg_catalog | pg_get_function_arg_default | text             | oid, integer
       | func |
+------------+-----------------------------+------------------+---------------------+------+
(1 row)