doc: Add note about postgres_fdw.application_name.

doc: Add note about postgres_fdw.application_name.

postgres_fdw.application_name can be any string of any length
and contain even non-ASCII characters.  However when it's passed
to and used as application_name in a foreign server, it's truncated
to less than NAMEDATALEN characters and any characters
other than printable ASCII ones in it will be replaced with question
marks. This commit adds these notes into the docs.

Author: Hayato Kuroda
Reviewed-by: Kyotaro Horiguchi, Fujii Masao
Discussion: https://postgr.es/m/TYCPR01MB5870D1E8B949DAF6D3B84E02F5F29@TYCPR01MB5870.jpnprd01.prod.outlook.com