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
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/6f85b317d6cc4638dc88175ec4721114dab42153

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

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