Information about controlling data-partiton size is missing

The app_flashlib_example accesses the data-partition. Therefore non-zero data partition has to be created when flashing the app_flashlib_example.
The data partition size is flash size - boot_partition_size (parameter to xflash)
This should be documented.

Also:
Executing The Example Application (Command Line):
should be changed to
Compiling The Example Application (Command Line):
