Clarify bits_per_word override documentation

Update documentation for read, write, and transfer methods to indicate
that the default value will fall back to the underlying bits_per_word
setting, not 8 as the documentation previously suggested.

Fixes #2

Author: tomstokes

spi.py

@@ -369,8 +369,8 @@ def write(self, data, speed=0, bits_per_word=0, delay=0):
             data: List of words to write
             speed: Optional temporary bitrate override in Hz. 0 (default)
                 uses existing spidev speed setting.
-            bits_per_word: Optional temporary bits_per_word override. 0 (
-                default) is equivalent to 8 bits per word.
+            bits_per_word: Optional temporary bits_per_word override. 0
+                (default) will use the current bits_per_word setting.
             delay: Optional delay in usecs between sending the last bit and
                 deselecting the chip select line. 0 (default) for no delay.
         """
@@ -390,8 +390,8 @@ def read(self, length, speed=0, bits_per_word=0, delay=0):
             length: Integer count of words to read
             speed: Optional temporary bitrate override in Hz. 0 (default)
                 uses existing spidev speed setting.
-            bits_per_word: Optional temporary bits_per_word override. 0 (
-                default) is equivalent to 8 bits per word.
+            bits_per_word: Optional temporary bits_per_word override. 0
+                (default) will use the current bits_per_word setting.
             delay: Optional delay in usecs between sending the last bit and
                 deselecting the chip select line. 0 (default) for no delay.
 
@@ -413,8 +413,8 @@ def transfer(self, data, speed=0, bits_per_word=0, delay=0):
             data: List of words to transmit
             speed: Optional temporary bitrate override in Hz. 0 (default)
                 uses existing spidev speed setting.
-            bits_per_word: Optional temporary bits_per_word override. 0 (
-                default) is equivalent to 8 bits per word.
+            bits_per_word: Optional temporary bits_per_word override. 0
+                (default) will use the current bits_per_word setting.
             delay: Optional delay in usecs between sending the last bit and
                 deselecting the chip select line. 0 (default) for no delay.