Document parameter "data" of i2c.mem_read() more clear

Hi,

i would like to add a little clarification to the parameter "data" of i2c.mem_read(): I misunderstood 

        ``data`` can be an integer or a buffer to read into

as "i can give a integer variable to read a integer into" .  This pull-request adds the following clarification:

         ``data`` can be an integer (number of bytes to read) or a buffer to read into

Thanks for your great work!

Best wishes,
Matthias