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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master

Details
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https://git.postgresql.org/pg/commitdiff/daf3d99d2b8bebb3361163a11ef3d232002127c9

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