pgsql: Allow setting effective_io_concurrency even on unsupported syste

Allow setting effective_io_concurrency even on unsupported systems

This matches the behavior of other parameters that are unsupported on
some systems (e.g., ssl).

Also document the default value.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/7feaccc21791d9d745bac013e5e0396bdde2d81c

Modified Files
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doc/src/sgml/config.sgml     |    4 ++++
src/backend/utils/misc/guc.c |    4 ----
src/bin/initdb/initdb.c      |    6 ++++++
3 files changed, 10 insertions(+), 4 deletions(-)