pgsql: Doc: update our docs about kernel IPC parameters on *BSD.

Doc: update our docs about kernel IPC parameters on *BSD.

runtime.sgml said that you couldn't change SysV IPC parameters on OpenBSD
except by rebuilding the kernel.  That's definitely wrong in OpenBSD 6.x,
and excavation in their man pages says it changed in OpenBSD 3.3.

Update NetBSD and OpenBSD sections to recommend adjustment of the SEMMNI
and SEMMNS settings, which are painfully small by default on those
platforms.  (The discussion thread contemplated recommending that
people select POSIX semaphores instead, but the performance consequences
of that aren't really clear, so I'll refrain.)

Remove pointless discussion of SEMMNU and SEMMAP from the FreeBSD
section.  Minor other wordsmithing.

Discussion: https://postgr.es/m/27582.1546928073@sss.pgh.pa.us

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8fd909436a44f5a3dd20c0c0d5bc0b5cbadd186e

Modified Files
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doc/src/sgml/runtime.sgml | 62 ++++++++++++++++++++++++++++++++---------------
1 file changed, 43 insertions(+), 19 deletions(-)