pgsql: pg_upgrade doc: link mode additions

pg_upgrade doc: link mode additions

Mention that link mode uses less disk space, and uses junction points on
Windows.

Backpatch to 9.3.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/85fa8293ad5621eaa747fbb62681b40d1b15dfa7

Modified Files
--------------
doc/src/sgml/pgupgrade.sgml | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

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