pgsql: Document that Solaris can't use effective_io_concurrency because

Log Message:
-----------
Document that Solaris can't use effective_io_concurrency because of an
ineffective posix_fadvise().

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.213 -> r1.214)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.213&r2=1.214)