pgsql: doc: Correct the description of server behavior after an OOM kil

doc: Correct the description of server behavior after an OOM kill

The documentation stated that existing connections would continue to
function normally after the OOM killer terminated the postmaster.
In fact, those connections eventually terminate when server processes
detect the postmaster's death.

Clarify that the OOM killer can terminate either the postmaster or a
child process. If the postmaster is killed, the server must be restarted.
If a child process is killed, PostgreSQL typically terminates the
remaining connections and either recovers automatically or shuts down,
depending on restart_after_crash.

Backpatch to all supported versions.

Author: Takeshi Ideriha <iderihatakeshi(at)gmail(dot)com>
Reviewed-by: Manuel Reyes Bravo <manuelreyesbravo(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/ce269321-a7f4-48f4-a9ca-a4a4983b9fd6@gmail.com
Backpatch-through: 14

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/43b0f718af0776588844effb2b71d353b219483b

Modified Files
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doc/src/sgml/runtime.sgml | 20 +++++++++++++-------
1 file changed, 13 insertions(+), 7 deletions(-)