pgsql: Document why Win32 loops over rename/unlink are necessary.
Log Message:
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Document why Win32 loops over rename/unlink are necessary.
Modified Files:
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    pgsql/src/port:
        dirmod.c (r1.38 -> r1.39)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/dirmod.c.diff?r1=1.38&r2=1.39)