parallel pg_restore and supported input format

Hi,

I found the document says that parallel pg_restore supports only
the custome archive format, as follows, but this is not true. Now,
it supports also the directory archive format.

> Only the custom archive format is supported with this option.

Also I found the following description about the input directory
of pg_dump is not true. If the target directory is empty, pg_dump
can continue its process even if the directory exists.

> In this case the directory is created by pg_dump and must not exist before.

The attached patch fixes these wrong descriptions.

Regards,

--
Fujii Masao