Docs: Add warnig re block protocol methods.

eeprom/i2c/I2C.md

@@ -216,6 +216,9 @@ These are provided by the base class. For the protocol definition see
 [the pyb documentation](http://docs.micropython.org/en/latest/library/uos.html#uos.AbstractBlockDev)
 also [here](http://docs.micropython.org/en/latest/reference/filesystem.html#custom-block-devices).
 
+These methods exist purely to support the block protocol. They are undocumented:
+their use in application code is not recommended.
+
 `readblocks()`  
 `writeblocks()`  
 `ioctl()`

eeprom/spi/SPI.md

@@ -209,6 +209,9 @@ These are provided by the base class. For the protocol definition see
 [the pyb documentation](http://docs.micropython.org/en/latest/library/uos.html#uos.AbstractBlockDev)
 also [here](http://docs.micropython.org/en/latest/reference/filesystem.html#custom-block-devices).
 
+These methods exist purely to support the block protocol. They are undocumented:
+their use in application code is not recommended.
+
 `readblocks()`  
 `writeblocks()`  
 `ioctl()`

flash/FLASH.md

@@ -271,6 +271,9 @@ These are provided by the base class. For the protocol definition see
 [the pyb documentation](http://docs.micropython.org/en/latest/library/uos.html#uos.AbstractBlockDev)
 also [here](http://docs.micropython.org/en/latest/reference/filesystem.html#custom-block-devices).
 
+These methods exist purely to support the block protocol. They are undocumented:
+their use in application code is not recommended.
+
 `readblocks()`  
 `writeblocks()`  
 `ioctl()`

fram/FRAM.md

@@ -187,6 +187,9 @@ These are provided by the base class. For the protocol definition see
 [the pyb documentation](http://docs.micropython.org/en/latest/library/uos.html#uos.AbstractBlockDev)
 also [here](http://docs.micropython.org/en/latest/reference/filesystem.html#custom-block-devices).
 
+These methods exist purely to support the block protocol. They are undocumented:
+their use in application code is not recommended.
+
 `readblocks()`  
 `writeblocks()`  
 `ioctl()`

fram/FRAM_SPI.md

@@ -179,6 +179,9 @@ These are provided by the base class. For the protocol definition see
 [the pyb documentation](http://docs.micropython.org/en/latest/library/uos.html#uos.AbstractBlockDev)
 also [here](http://docs.micropython.org/en/latest/reference/filesystem.html#custom-block-devices).
 
+These methods exist purely to support the block protocol. They are undocumented:
+their use in application code is not recommended.
+
 `readblocks()`  
 `writeblocks()`  
 `ioctl()`

spiram/SPIRAM.md

@@ -163,6 +163,9 @@ These are provided by the base class. For the protocol definition see
 [the pyb documentation](http://docs.micropython.org/en/latest/library/uos.html#uos.AbstractBlockDev)
 also [here](http://docs.micropython.org/en/latest/reference/filesystem.html#custom-block-devices).
 
+These methods exist purely to support the block protocol. They are undocumented:
+their use in application code is not recommended.
+
 `readblocks()`  
 `writeblocks()`  
 `ioctl()`