pgsql: Add comment to explain why PGReserveSemaphores() is called early

Add comment to explain why PGReserveSemaphores() is called early

Before commit e25626677f, PGReserveSemaphores() had to be called
before SpinlockSemaInit() because spinlocks were implemented using
semaphores on some platforms (--disable-spinlocks). Add a comment
explaining that.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAExHW5seSZpPx-znjidVZNzdagGHOk06F+Ds88MpPUbxd1kTaA@mail.gmail.com
Backpatch-to: 18

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/75ec47c38bcf8777164cf91885eb907eb3fc369b

Modified Files
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src/backend/storage/ipc/ipci.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)