update doc for file options in openfpga bitstream
@@ -124,7 +124,9 @@ FPGA-Bitstream
.. option:: build_fabric_bitstream
- Reorganize the bitstream database for a specific FPGA fabric
+ Build a sequence for every configuration bits in the bitstream database for a specific FPGA fabric
+
+ - ``--file`` or ``-f`` Output the fabric bitstream to an plain text file (only 0 or 1)
- ``--verbose`` Show verbose log
