pw_sync_freertos: Update InterruptSpinLock docs
pw_sync_freertos: Update InterruptSpinLock docs

Small update for InterruptSpinLock clarifying that InterruptSpinLock
cannot be used in interrupts with priority above
configMAX_SYSCALL_INTERRUPT_PRIORITY.

Change-Id: I57349afc5e6fe918ae4f62ccbbd708b0a03536e8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/108091
Reviewed-by: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>
Commit-Queue: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>

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