doc: Move synchronized_standby_slots to "Primary Server" section.

doc: Move synchronized_standby_slots to "Primary Server" section.

synchronized_standby_slots is defined in guc_parameter.dat as part of
the REPLICATION_PRIMARY group and is listed under the "Primary Server"
section in postgresql.conf.sample. However, in the documentation
its description was previously placed under the "Sending Servers" section.

Since synchronized_standby_slots only takes effect on the primary server,
this commit moves its documentation to the "Primary Server" section to
match its behavior and other references.

Backpatch to v17 where synchronized_standby_slots was added.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Shinya Kato <shinya11.kato@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwE_LwgXgCrqd08OFteJqdERiF3noqOKu2vt7Kjk4vMiGg@mail.gmail.com
Backpatch-through: 17