Clarify: default precision on timestamps is 6

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/16/datatype-datetime.html
Description:

https://www.postgresql.org/docs/current/datatype-datetime.html says:

> time, timestamp, and interval accept an optional precision value p which
specifies the number of fractional digits retained in the seconds field. By
default, there is no explicit bound on precision. The allowed range of p is
from 0 to 6.

I think it would be more accurate to say "By default, the bound on precision
is 6 (that is, microsecond precision). The allowed range of p is from 0 to
6. Values with greater precision than the type allows will be rounded to its
maximum precision, and values with less precision than the type allows will
be stored as given."

The following experiment confirms that 'timestamp with time zone' behaves
exactly like 'timestamp(6) with time zone'; both of them round any precision
beyond microseconds, and neither returns timestamps with greater precision
than the value that was inserted.

```sql
# CREATE TABLE tstest (a timestamp with time zone, b timestamp(6) with time
zone, c timestamp(2) with time zone);
CREATE TABLE
# \d tstest
                         Table "public.tstest"
 Column |            Type             | Collation | Nullable | Default
--------+-----------------------------+-----------+----------+---------
 a      | timestamp with time zone    |           |          |
 b      | timestamp(6) with time zone |           |          |
 c      | timestamp(2) with time zone |           |          |

-- microsecond precision
# INSERT INTO tstest (a, b, c) VALUES ('2024-01-01 00:00:00.123456Z',
'2024-01-01 00:00:00.123456Z', '2024-01-01 00:00:00.123456Z');
INSERT 0 1

-- nanosecond precision
# INSERT INTO tstest (a, b, c) VALUES ('2024-01-01 00:00:00.123456789Z',
'2024-01-01 00:00:00.123456789Z', '2024-01-01 00:00:00.123456789Z');
INSERT 0 1

-- millisecond precision
# INSERT INTO tstest (a, b, c) VALUES ('2024-01-01 00:00:00.123Z',
'2024-01-01 00:00:00.123Z', '2024-01-01 00:00:00.123Z');
INSERT 0 1

-- second precision
# INSERT INTO tstest (a, b, c) VALUES ('2024-01-01 00:00:00Z', '2024-01-01
00:00:00Z', '2024-01-01 00:00:00Z');
INSERT 0 1

--- results
# SELECT * FROM tstest;
               a               |               b               |
c
-------------------------------+-------------------------------+---------------------------
 2024-01-01 00:00:00.123456+00 | 2024-01-01 00:00:00.123456+00 | 2024-01-01
00:00:00.12+00
 2024-01-01 00:00:00.123457+00 | 2024-01-01 00:00:00.123457+00 | 2024-01-01
00:00:00.12+00
 2024-01-01 00:00:00.123+00    | 2024-01-01 00:00:00.123+00    | 2024-01-01
00:00:00.12+00
 2024-01-01 00:00:00+00        | 2024-01-01 00:00:00+00        | 2024-01-01
00:00:00+00
(4 rows)
```