pgsql: Add note about risks involved in replaying CREATE TABLESPACE

Log Message:
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Add note about risks involved in replaying CREATE TABLESPACE commands
from WAL. A couple other grammatical improvements too.

Modified Files:
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pgsql/doc/src/sgml:
backup.sgml (r2.59 -> r2.60)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml.diff?r1=2.59&r2=2.60)