[Documentation] Workaround for errata RP2040-E12 in datasheet incomplete

[vbargsten]

It states that

Instead of checking READ_ADDR or WRITE_ADDR to monitor the progress of a transfer sequence, check
TRANS_COUNT

However, TRANS_COUNT is zero (in contrast to READ_ADDR) after the dma transfer has been aborted, for example. It is unclear how to determine the number of transferred bytes after an abort. The transfer could be paused (disabled), TRANS_COUNT read, and then aborted. But in this case it remains unclear what has to be done to properly account for in-flight data.