Clarify BASEPRI and NVIC interaction

Co-authored-by: Henrik Tjäder <henrik@tjaders.com>
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@ -28,8 +28,10 @@ This implementation is covered in depth by Chapter 4.5 of this book.
## Source Masking
Since there is no hardware support for a priority ceiling, RTIC must instead rely on the Nested
Vectored Interrupt Controller (NVIC) present in the core architecture. Consider Figure 1 below,
Without a `BASEPRI` register which allows for directly setting a priority ceiling in the Nested
Vectored Interrupt Controller (NVIC), RTIC must instead rely on disabling (masking) interrupts.
Consider Figure 1 below,
showing two tasks A and B where A has higher priority but shares a resource with B.
#### *Figure 1: Shared Resources and Source Masking*