pgsql: Mention that pg_upgrade requires write permission in the current

Mention that pg_upgrade requires write permission in the current
directory.

Per report from Harald Armin Massa.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=798cbc015b6512b54a41bf3d40fd235322125292

Modified Files
--------------
doc/src/sgml/pgupgrade.sgml | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)