pgsql: Document that max_worker_processes must be high enough in standb

Document that max_worker_processes must be high enough in standby.

The setting values of some parameters including max_worker_processes
must be equal to or higher than the values on the master. However,
previously max_worker_processes was not listed as such parameter
in the document. So this commit adds it to that list.

Back-patch to 9.4 where max_worker_processes was added.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/1ea5ce5c5f204918b8a9fa6eaa8f3f1374aa8aec

Modified Files
--------------
doc/src/sgml/high-availability.sgml |    5 +++++
src/backend/access/transam/xlog.c   |    4 ++++
2 files changed, 9 insertions(+)