i2c porting guide: add i2c_set_clock_stretching

Author: maciejbocianski

docs/porting/target/i2c.md

@@ -23,6 +23,9 @@ The interface is made up of two lines for all communication:
  - Sets the frequency to use for the transfer.
  - Returns the actual frequency used.
  - Must leave all other configuration unchanged.
+- `i2c_set_clock_stretching`:
+ - Enables or disables clock stretching for the peripheral when in slave mode.
+ - Does nothing when called in master mode.
 - `i2c_timeout`:
  - Sets the transmision timeout to use for the following blocking transfers.
  - If the timeout is not set, the default timeout is used.