pgsql: doc: Add note about the server name of postgres_fdw_get_connecti

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ee79a548e746da9a99df0cac70a3ddc095f2829a

Modified Files
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doc/src/sgml/postgres-fdw.sgml | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)