docs on read() need additional information

[2bndy5]

I noticed that the nRF24L01 will return additional data from other payloads in the RX FIFO when the len argument is larger than the next payload in the RX FIFO (in consecutive order of course). If len is greater than the last payload available in the RX FIFO, then buf is padded with the last available byte accordingly. This functionality can be useful for fetching the entire RX FIFO contents in 1 SPI transaction. Since this behavior is embedded in the nRF24L01 firmware, the library should not try to compensate.

To be clear, payloads are not removed from the RX FIFO unless the entire payload is fetched. Also, if len is less than the next available payload, then the payload will remain until entirely read from the RX FIFO. According to the datasheet, all attempts to read from the RX FIFO start at the first byte in the first available payload.