Improved comments and fixed non-critical typo

Thanks to https://github.com/integerbasic for improving the comments and fixing a typo. Nothing has changed in functionality or the API.

1. Fix typo in comment; GCLK 1 is used; not GCLK 0.
2. Change macro name (CPU_HZ -> GCLK1_HZ) to better match what it refers to.
3. Add some comments to explain assumptions about device vendor clock setup.

v1.1

Added stop & restart functionality by merging the fork https://github.com/sennert/SAMD51_InterruptTimer by Sennert. Thanks Sennert. Note: I did not personally test the new functions myself, so I'm relying on Sennert to have done things right :).

v1.0

Interrupt service routine timer library for an Atmel SAMD51 microprocessor board, like Adafruit M4 Metro/Feather/ItsyBitsy Express.