doc: Enhance documentation for postgres_fdw_get_connections() output columns.

doc: Enhance documentation for postgres_fdw_get_connections() output columns.

The documentation previously described the output columns of
postgres_fdw_get_connections() in text format, which was manageable
for the original two columns. However, upcoming patches will add
new columns, making text descriptions less readable.

This commit updates the documentation to use a table format,
making it easier for users to understand each output column.

Author: Fujii Masao, Hayato Kuroda
Reviewed-by: Hayato Kuroda
Discussion: https://postgr.es/m/d04aae8d-05f5-42f4-a263-b962334d9f75@oss.nttdata.com