Documentation: i2c: slave: describe buffer problems a bit better

bytes came up. Such an extension might be possible, usefulness is unclear at

this time of writing. Some points to keep in mind when using buffers:

* Buffers should be opt-in and slave drivers will always have to support

  byte-based transactions as the ultimate fallback because this is how the

  majority of HW works.

* For backends simulating hardware registers, buffers are not helpful because

  on writes an action should be immediately triggered. For reads, the data in

  the buffer might get stale.

* Buffers should be opt-in and backend drivers will always have to support

  byte-based transactions as the ultimate fallback anyhow because this is how

  the majority of HW works.

* For backends simulating hardware registers, buffers are largely not helpful

  because after each byte written an action should be immediately triggered.

  For reads, the data kept in the buffer might get stale if the backend just

  updated a register because of internal processing.

* A master can send STOP at any time. For partially transferred buffers, this

  means additional code to handle this exception. Such code tends to be