doc: Add note about the server name of postgres_fdw_get_connections() returns.

Previously the document didn't mention the case where
postgres_fdw_get_connections() returns NULL in server_name column.
Users might be confused about why NULL was returned.

This commit adds the note that, in postgres_fdw_get_connections(),
the server name of an invalid connection will be NULL if the server is dropped.

Suggested-by: Zhijie Hou
Author: Bharath Rupireddy
Reviewed-by: Zhijie Hou, Fujii Masao
Discussion: https://postgr.es/m/e7ddd14e96444fce88e47a709c196537@G08CNEXMBPEKD05.g08.fujitsu.local

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