pgsql: doc: reformat GRAPH_TABLE examples

doc: reformat GRAPH_TABLE examples

The GRAPH_TABLE examples in ddl.sgml and queries.sgml were written
as single long lines, making their structure harder to read in the
generated documentation.

Reformat these examples so that the graph name, MATCH clause, and
COLUMNS clause appear on separate lines, making them easier to read.

Author: Koshino Taiki <koshino(at)sraoss(dot)co(dot)jp>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/OS9P286MB64860BD6CD6D4B4B0E1CEDE894A32@OS9P286MB6486.JPNP286.PROD.OUTLOOK.COM
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/a20d3e35f2c00938c5329cbf570160184f4f0a69

Modified Files
--------------
doc/src/sgml/ddl.sgml | 12 +++++++++---
doc/src/sgml/queries.sgml | 13 +++++++++----
2 files changed, 18 insertions(+), 7 deletions(-)