[PATCH] Fix for documentation of timestamp type

Hello.

Currently doc/src/sgml/datatype.sgml states:

```
When <type>timestamp</> values are stored as eight-byte integers
(currently the default), microsecond precision is available over
the full range of values. When <type>timestamp</> values are
stored as double precision floating-point numbers instead (a
deprecated compile-time option), the effective limit of precision
might be less than 6. <type>timestamp</type> values are stored as
seconds before or after midnight 2000-01-01. [...]
```

It gives a wrong impression that by default timestamp is stored as a
number of seconds after midnight 2000-01-01 in a eight-byte integer. In
fact timestamp is stored in MICROseconds, not seconds. For instance,
2016-12-12 16:03:14.643886 is represented as number 534873794643886:

```
$ echo "select relfilenode from pg_class where relname = 'tst';" | psql
relfilenode
-------------
16431
(1 row)

$ find /path/to/data/dir -type f -name 16431
[...]

$ hexdump -C path/to/found/table/segment
00000000 00 00 00 00 08 13 10 03 00 00 00 00 1c 00 e0 1f
00000010 00 20 04 20 00 00 00 00 e0 9f 40 00 00 00 00 00
00000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
00001fe0 3c 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00001ff0 01 00 01 00 00 09 18 00 ae 87 87 02 77 e6 01 00

$ python
>>> "{:x}".format(534873794643886)
'1e677028787ae'
```

'ae 87 87 02 77 e6 01 00' is exactly what is physically stored on disk.
You can calculate current year from number 534873794643886 like this:

```
>>> int(2000 + 534873794643886 / 1000 / 1000 / 60 / 60 / 24 / 365.2425)
2016
```

I suggest to rewrite the documentation a bit to make it more clear that
by default timestamp is stored in microseconds. Corresponding patch is
attached.

--
Best regards,
Aleksander Alekseev