pgsql: Update our documentation concerning where to create data directo

Update our documentation concerning where to create data directories.

Although initdb has long discouraged use of a filesystem mount-point
directory as a PG data directory, this point was covered nowhere in the
user-facing documentation.  Also, with the popularity of pg_upgrade,
we really need to recommend that the PG user own not only the data
directory but its parent directory too.  (Without a writable parent
directory, operations such as "mv data data.old" fail immediately.
pg_upgrade itself doesn't do that, but wrapper scripts for it often do.)

Hence, adjust the "Creating a Database Cluster" section to address
these points.  I also took the liberty of wordsmithing the discussion
of NFS a bit.

These considerations aren't by any means new, so back-patch to all
supported branches.

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/7bdf6d0440697619cea61841cd1432337b151b47

Modified Files
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doc/src/sgml/runtime.sgml |   79 ++++++++++++++++++++++++++++++++-------------
1 file changed, 57 insertions(+), 22 deletions(-)