Document unavailable parameters in some configurations

Add a note to user-facing parameters that can be removed completely
(and not just empty) by #ifdef's depending on build configuration.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=8b4d3d44dc7d64aaafd2269d3da8d2873137c691

Modified Files
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doc/src/sgml/config.sgml |    9 ++++++---
1 files changed, 6 insertions(+), 3 deletions(-)

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